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Reinforced AR-15 Lower Receiver

by HaveBlue, published

Reinforced AR-15 Lower Receiver by HaveBlue Sep 21, 2011

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I've been working on this for several months and after seeing a stock AR lower uploaded yesterday (with no attribution provided), I decided to post mine in order to provide proper upstream credit where it is due. This receiver is based on the IGES file from Justin Halford's superb CNC Gunsmithing site: http://www.cncguns.com/downloads.html

[edit] I see KingLudd has now credited Justin's site as the origin of the solid model - thank you.


If you have access to a CAD package, I highly recommend working from the original IGES file and not an STL if you want to print a lower receiver. Firearms require a good deal more precision than a Bre Pettis bobblehead, and working from a proper CAD model will allow you to tweak hole diameters and add reinforcement in critical areas. In this case, I opted to add extra support to the front takedown pin lugs, added an integral trigger guard, and reinforced the area around the bolt hold. However, I'm still skeptical of how well the bolt hold crosspin holes will fare and will test without that feature to start with.

Justin's original IGES file actually has the buffer tube threads as modeled features, so I output the STL at a very high resolution (hence the 46 meg file). Much to my delight, when I printed the receiver on my Stratasys, a buffer tube could be screwed right into the rear of the receiver with no touchup of the threads needed (a good thing, as the special 1-3/16"-16 tap for the threads is rather expensive).

As this is a rather complex part, I recommend printing it out at 75% scale first (you can see my original 75% stock lower next to the final full size reinforced lower in one of the pictures). You'll save a lot of time and material this way and get a good idea of how well features will be preserved on a full size print. I used PP3DP UP! filament for the 75% model and switched to black Bolson ABS for the full print. I'd love to try a polycarbonate/ABS blend for this part, but I have no idea if my printer will handle such material.

I have not yet tried fitting the receiver with the trigger group and other components - I may need to ream some of the holes and then sleeve with brass bushings to maintain the needed accuracy. However, I'm very pleased with the receiver thus far and have a .22 conversion kit that I'll be using once I'm ready for live fire tests. For zombie extermination, I hope to use crank's excellent AR magazine: http://www.thingiverse.com/thing:11636

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anyone have a link to get the .stl files?

I posted them to my blog after Thingiverse removed them: http://haveblue.org/?p=1530

However, there are far more refined versions available in the FOSSCAD repository: https://github.com/maduce/fosscad-repo

On your website you said people had a more refined version of the lower. Where can you find it?

Do you know where i can find these files? http://www.thingiverse.com/thing:719608
I found glocks on grabcad but Cura wont upload because they are not .stl or .amf. thanks. please attach link if you find one.

Most items on GrabCad are going to be SolidWorks, Inventor, or IGES files - you'll need to use an appropriate CAD program to open and convert them into a triangle mesh.

Thanks. it is kinda sad they removed this..

True, but the FOSSCAD community was born as a result and serves as an excellent repository of files and knowledge. All my files are in the FOSSCAD repo on github (https://github.com/maduce/fosscad-repo) and I reposted them on my blog as well.

where is the picture ?

7777773 - in reply to

Thingiverse deleted everything claiming they were "illegal" - they're OK with drug paraphernalia, but not civil rights. It's their website, their rules. The files are available on other more open model-sharing sites, thingiverse was co-opted when makerbot sold out, and they have been doing things like censoring and illegally claiming patents on user submissions, so its best to simply avoid this site.

Why did they have to censor this?

hey if you printed this with abs, then did an acetone vapor bath, it would greatly improve the strength of the reciever.

Look up DEFCAD. Enjoy!

“Thousands of years ago tribes of human beings suffered great privations
in the struggle to survive. In this struggle it was important not only
to be able to handle a club, but also to possess the ability to think
reasonably, to take care of the knowledge and experience garnered by the
tribe, and to develop the links that would provide cooperation with
other tribes. Today the entire human race is faced with a similar test.
In infinite space many civilizations are bound to exist, among them
civilizations that are also wiser and more "successful" than ours. I
support the cosmological hypothesis which states that the development of
the universe is repeated in its basic features an infinite number of
times. In accordance with this, other civilizations, including more
"successful" ones, should exist an infinite number of times on the
"preceding" and the "following" pages of the Book of the Universe. Yet
this should not minimize our sacred endeavors in this world of ours,
where, like faint glimmers of light in the dark, we have emerged for a
moment from the nothingness of dark unconsciousness of material
existence. We must make good the demands of reason and create a life
worthy of ourselves and of the goals we only dimly perceive.”

Andrei Sakharov

Nice to see how cowardly Thingiverse is. no freedom of speech??

Yay, first going closed source on the Software and the Makerbot2, then changing the ToS of Thingiverse and now deleting files. Congrats, Makerbot, Congrats Bre.

F... you Makerbot, for jumping on this national panic bandwagon to ban all the evil "@ssualt W3apons" and removing this object. I cannot support a company that is in line with a political agenda built on the backs of the dead. You have lost a customer for life. I wish nothing but the worst for your company.

F#$k You Makerbot, for jumping on this national panic bandwagon to ban all the evil "@ssualt w3apons" and removing this object. I cannot support a company that is in line with a political agenda built on the backs of the dead. You have lost a customer for life. I wish nothing but the worst for your company.

Ethier I can't see the files or Thingiverse took them down. They were up about an hour earlier though...???

Unfortunately, Thingiverse removed my files for the lower as well as the trigger guard.  I got this message:

Regarding Thingiverse Content: Reinforced AR-15 Lower Receiver by HaveBlue
AR-15 Enhanced Trigger Guard by HaveBlue
a Thingiverse User, you have agreed to accept and abide by our Terms of
Use (the "Terms").  Our Acceptable Use Policy in section 3.3 of the
Terms describes use of the Site "to collect, upload, transmit, display
or distribute any User Content... (ii) that...promotes illegal
activities or contributes to the creation of weapons..." as a violation.

In exercising our policy enforcement discretion, we have decided to remove the above-captioned content as of today.Please contact me with any questions.

-- Rich McCarthyCorporate CounselMakerBot Industries LLC

87 3rd Avenue, Brooklyn NY 11217347-391-2736


That is a load of junk in my opinion. Do you stil have the files?

Post them to GrabCad. It's like Thingiverse without the political agenda.

I still have the files somewhere. There were over 20,000 downloads of the AR lower STL, so there are plenty of copies floating around (as far as I could tell, it was the most downloaded item on Thingiverse). I'll see if I can post what I have to GrabCad, though what's to stop them from taking down items on a whim?

I called Mr. McCarthy when I got the email notice, and noted the fact that 1) I had uploaded the AR lower before the changes to the terms of service, and 2) that my trigger guard was hardly a weapon and could be used for paintball and airsoft. His response was essentially, 'our sandbox, our rules, and we can change the rules ant any time'.

Also, this page is completely hidden from anyone that hasn't already participated on the page... Try looking it up from a browser when not logged in.
I may need to add a Grabcad link to all of my other things that haven't yet been declared sharp or scary or frightening.

I just found GrabCad about 5 seconds before you replied to that comment, but yes. GrabCad files would b awesome. And a .dwg file would be 100x better.

Cool stuff HaveBlue.

Can you (or anyone else for that matter) comment on the Bolson Materials?  I've got a uPrint SE Plus and am thinking of trying some Bolson B30ABS.  So far, you're the only project I've come across that mentions Bolson Materials.  Specifically, I'm wondering how interchangeable is their material and does it give you a consistent print every time?

Judging by what I see here, I'm thinking they offer a pretty good product. 

First off, I'm not sure if Bolson is able to supply correct EEPROMs for the uPrint series machines (from what I hear, uPrints use a variant EEPROM that includes MAC authentication, so normal EEPROM re-write hacks won't work (but it Bolson can supply you with a working EEPROM along with the filament, it's no big deal)).

As far as the material itself goes, the standard Bolson ABS seems ever-so-slightly 'rubbery' compared to standard Stratasys ABS.  However, I understand that Bolson's B30ABS is exactly the same as Stratasys ABSplus (which itself is SABIC MG94 to the best of my knowledge).  My own next revision of the AR lower will be made out of MG94, which should certainly provide improved qualities over the standard ABS.

Thanks HaveBlue,

I've already spoken to Bolson and they are working on the uPrint chips.  I need a custom color run of material for a consumer product, and the price difference between OEM and Bolson is so great that I can buy spools of OEM material, throw away the filament an use the chips with the custom color run Bolson material and still be money ahead.  (Not to mention that the min order is about 5x less).

Thanks again and good luck!  Looking forward to seeing how your MG94 turns out.

Sounds good!  You'd easily be able to sell the OEM material for a few bucks on Ebay, so don't just toss the spools!  A number of us old Stratasys users (or Up! users for that matter) would happily buy them at a discount.

Good thought - what's your favorite color?  They're all the same price from the OEM.

By the way - who do you use to custom extrude your material from Sabic?  If Bolson can't get the chips fixed, why not skip the middleman?

I think black is probably the most desired color of all the Stratasys shades.  Note that they just came out with Ultem 9085 in black just a few days ago (and released Insight v8.1.1 to accomodate it) - the standard natural shade of 9085 did look a bit like vomit  :-)

I used a local plastic extrusion shop to run MG94 resin into filament - depending on the cost and quantity, it may very well be worthwhile to cut out the middleman and get the filament made-to-order locally!  This stuff isn't rocket science, after all!

Is there a model that does 5.56?

It's been tested with .223 Rem, but there's enough flex in the buffer tower to make cycling problematic.  I'll try thickening the buffer tower and doing a print with methyl methacrylate ABS to see if that helps any.  Even so, the best route is to use oryhara's thumbhole buttstock so that the buffer tube is fully supported and remains as co-axial as possible with the reciprocating bolt/carrier group:  http://www.thingiverse.com/thing:27548http://www.thingiverse.com/thi...

Of course, if you reload, you could also try adjusting the powder charge to give the system 'just enough' gas to cycle (oh, how I wish the AR-15 had an adjustable gas block...)

most oils dissolve abs plastic. what lube do you use to lube your printed abs ar 15? vegetable oil? silicon grease? thanks :0)

 I haven't bothered to lubricate the lower yet, but I'd use plain old Breakfree CLP (what I use on my uppers), as it doesn't attack plastic.

If I were really worried about oil attacking the ABS, I'd probably just use graphite powder for the internals on the lower.

Hi HaveBlue. Nick Bilton here, I'm a technology columnist with The New York Times. I've written extensively about 3-D printing. I have a few questions about this particular part and would love to chat more. [email protected] - thanks! Nick.

Not familer with the ABS material. From what I read the burnout process seems to have what could be two potential drawbacks for hobbyists like me. Specifically the need for very high temperatures to melt the ABS and seperate rinse process that's not done with wax. I also saw that there were issues with anticipated shrinkage that is greater than normally found in more traditional investment casting with wax molds.

 The process is certainly accessible to the hobbyist with PLA at least:  http://3dtopo.com/lostPLA/http://3dtopo.com/lostPLA/

Has anyone printed this lower receiver in wax for investment casting? I have access to a small aluminum foundry and would like to experiment with this.

Not to my knowledge, but you can use the ABS prints as masters just as you would with wax: http://www.fortus.com/~/media/Main/Files/Applications/AG-InvestmentCasting-0109.ashxhttp://www.fortus.com/~/media/...

what does the print need to be scaled by to be printed and what should the object infill layer height and number of shells be also should i use a raft and full support

The STL is in inches, so scale by 25.4 if you need mm.

Layer height and such is up to you, but make sure it's a fully filled solid if you intend to try using it.

so that is scaling it from what is is when it is downloaded in the box that says scale in replicator g enter 25.4

I don't use Replicator G, so I can't say how you might need to scale it in that software.

what orientation did you use considering if i am trying to make a functional part and what software did you use to print it

I used Insight, which is the software used on Stratasys machines. I aligned the magwell with the Z-axis for my print, but others have aligned the takedown pin holes with the Z axis. As I said, you'll have to determine for yourself which orientation to use based on which axis you can tolerate less strength in, how parts tend to warp on your machine, and what sort of dimensional accuracy each axis can achieve. If those are unknowns, you'll probably have to experiment with several prints to find the best orientation and do some test fitting with a LPK and upper.

also which way should the model be orientated for printing

Model orientation is entirely up to you and will depend on which features you want to retain the most accuracy on as well as the print envelope available. I recommend studying a receiver blueprint and looking up information on how to assemble parts into it so that you can determine which orientation is best for your needs. Keep in mind the stress points you'll encounter - my front takedown lugs have broken in use, so you might want to print on the side to keep it from splitting in that location. However, printing on the side means a lot more cleanup of support material from the magwell and FCG pocket. You'll need to determine for yourself what tradeoffs to make.

The biggest issue to be concerned with (assuming your accuracy is sufficient to build a functional part) is warping of the model - you may need to experiment with various build orientations to find one that suits your particular setup.

Dear HaveBlue.

I am not interested in discussing weapons legislation etc, but do you anticipate problems with the socalled "agreed user content" of this site?

From the "Acceptable Use Policy":
gt;....(a) You agree not to use the Site or Services to collect, upload, transmit, display, or distribute any User Content (i) that.......... contributes to the creation of weapons.....

I know that Thiingiverse let it slip early with rubberband guns and boomerangs but anyway :)


That's a good question - I uploaded the file back in September, when there was no mention of weapons in the Acceptable Use Policy. The terms were modified in February, I believe.

As such, I don't know if I'm in a gray area, if my upload is 'grandfathered' in, or if the lower receiver is considered to be only a weapon part, and not a full weapon as such. I suspect the Thingiverse admins are also unsure of exactly where the upload stands.

So maybe the addition to the Acceptable Use Policy reflects the outcome of the old poll regarding weapons on Thingiverse? So today, a comparable upload might not be possible because one would have to accept the updated Acceptable Use Policy when posting, but your upload is in the safe because it was uploaded under the old policy? Have you got any more models in stock to test if this is so? O:-)

If you have a closeup of the printed buffer tube threads - could you show it? I am curious to what detail is required to make working threads in that size. Currently my Makerbot TOM wont be able to.

I don't believe the poll results were ever released, and it's quite possible that they were of no consequence in the end. If you read Bre's blog post announcing that the terms of use had been changed (http://blog.thingiverse.com/2012/02/10/thingiverse-updates-terms-of-use-and-license-options/)http://blog.thingiverse.com/20..., he notes that lawyers crafted the document. As such, I'm guessing that the lawyers simply said "no weapons" regardless of what the poll results may have indicated.

I'm sure I could come up with a model to upload to 'test' the new policy, but I'm not about to. It was never my intent to violate any policies. I was simply curious to find out if a printed AR-15 lower could work, and I guessed that a lot of other people with an interest in hobbyist gunsmithing
would also be curious.

However, it doesn't appear that the policy is being enforced, as this object came after the policy change, and even has 'weapon' in the title: http://www.thingiverse.com/thing:19036http://www.thingiverse.com/thi... I'm wondering if the word 'weapons' in the terms is so narrowly defined in the minds of the
lawyers who drafted it that it winds up not applying to such items? In the end, it just opens back up the whole "what consititutes a weapon" debate that Zach had mentioned.

As for a closeup of the printed threads, you can sort of see them in this photo: http://haveblue.org/wordpress/wp-content/uplhttp://haveblue.org/wordpress/...
They're a little rougher up at the top due to overhang, but a buffer tube threaded in just fine.

Weapon - Victorian Knuckle

Michael - the issue was discussed last year quite a bit.

Scroll down to read up on the comments for this specific file from back when this receiver was posted nearly a year ago.... and this wasn't even the 1st AR receiver to be posted on this site - the first AR parts were posted a little over a year and a half ago.

Summing up: the issue has been settled for a very long time.

Thank you for your answer 7777773

I did look up on old discussions but I could not find a final answer e.g. from Thingiverse moderators. There was also mention of an old poll to decide what the users wanted but I could not find the result of that poll.


WOW, This is fantastic. Would one of the makerbots be good enuff for this. Is there any other forums were I can learn more about this topic. ie cad software running on linux. I am so excited.

I know a Replicator is big enough to make one, but I don't know about older models. oryhara is leading the way with a 2-piece lower that should allow the receiver to be printed on slightly smaller machines, as well as making it easier to tweak the buffer tube threads: http://rommie.digitalcrowbar.net/wordpress/http://rommie.digitalcrowbar.n...

So. It's time to go ahead and start filing paperwork on this. Trust me, an ounce of prevention is beeter than a poud of cure. What you are making here is the one piece of the weapon the ATF actually cares about - the lower receiver. It appears you don't have a serial on it either - that's a big fat no-no.

You need a type 7 FFL for manufacturing, and you will have to file for everyone of these you make and pay tax if you exceed a certain amount (50 IIRC) per year. Don't let the internet "freedom" yahoos fool you, if the ATF makes you a test case it will go very badly. Even if you prevail in court
, which is doubtful because you've publicly stated your intent is to manufacture a working weapon, you'll take a huge financial hit from the lawyers.

A serial number and other markings are only needed if the firearm is transferred down the road. Anyone can build one of these without a manufacturing FFL unless they're making them for the purpose of selling them.

"Even if you prevail in court, which is doubtful because you've publicly stated your intent is to manufacture a working weapon"

You're talking out of your backside. It's completely legal to make a working firearm in your garage with no paperwork and no permission from the US government. You do
need all of the paperwork you mention to make a working firearm with the intent to sell it, but that's not what's being discussed here.

I made the lower for personal use, with zero intent to sell or transfer the firearm. As such, I do not need a type 7 FFL, and the laws in my state do not require that it be engraved with a serial number.

This has been done thousands of times before by many other home gunsmiths.

Make some ak stuff instead.. :/

I've been pondering it, though I think a printed AK receiver is a bit less practical than one for an AR.

Love this, just printed it at 50% size, had some minor issues with the supports but once it was complete it just looked perfect.

Thank you, excellent work.

HB... hmmm... that's a familiar name from the past ;)

Great job on the article and blog!

Likewise, TW! Good to hear you're still around!

HB, done any work on making your own upper? I see a lot of different calibers coming out in uppers these days (the most intriguing to me being a 458 SOCOM - available in a subsonic load and just begging to be surpressed)... I think it might be interesting to create a table driven solid model so that you could select a caliber and regenerate the model based on those changes necessary...

TW, I haven't done anything with regards to making my own uppers (I still need to get around to machining the 0% lower forgings I have gathering dust, much less any work above the trigger group). However, the concept of a table-driven, parametric upper based on cartridge is a fantastic idea! Agreed, there's a lot of neat rounds out there, and a cartridge generated upper is really intriguing.

I have a 0% upper and lower as well. they are paper weights to me right now. You want them?

I'll pass on the paper weights - they would be little more than hunks of metal for me as well (just not enough time).

This is very interesting work and dialog gentlemen. Has anyone done this for the M1A1 receiver? I have the 2D but was hopeful of scoring big with the solid already being available...life would be good if it was in a format other than STL. Thanks for all the info and the links...I followed those and when I stopped I was wondering when it got dark outside!


This guy mills and tests a homemade plastic AR-15 lower.

Anyone know if this has been tested yet?

Not yet - I've slowly been cleaning up holes on the receiver to fit the internals.

Hey man! I was wondering if you had tried this out yet :)

Not yet, but I am getting closer - last week I started fitting it out with internals, and it's going well - will probably do a blog post on it.

this would be great if it didnt have the shelf inside.

Yes, but if I could afford a registered DIAS (or even better, had a Class 2 SOT), I'd just machine the blasted thing out of aluminum...

Actually, a plastic lower receiver is entirely plausible. I recall Cav Arms selling a single unit lower and A2 butt-stock made from plastic. The Lower actually has relatively little stress applied to it during operation.

Yep. PlumCrazy has been offering complete 5.56 plastic lowers for years.

Great print quality. Keep us updated with a range report!

Fantastic work, and a really great looking print.. Who will post the first M1911 I wonder? :)

A 1911 wouldn't be hard to print - Justin has the IGES file for the frame on his site, and you can even get a full SolidWorks model of the complete pistol from Rio Benson for $50 (http://riobenson.comhttp://riobenson.com). The solid model is superb - even if you don't have an interest in firearms, browsing through the feature tree to see how it was constructed teaches you a great deal of insight into how a pro approaches modeling a reasonably complex part.

However, given that the 1911 frame receives a lot more punishment than an AR-15 lower, it won't exactly be usable! It might serve as a reasonable master for investment casting, but even then the part would need a lot of cleanup.

I posted the plans for a m1911a1. I think if you converted it to 22 and sleeved the barrel it could be doable

Heck with sleeving the barrel - just use a Kimber or other .22 conversion. I'd still be a bit hesitant with a printed frame and rimfire ammo - I don't know that the slide stop would be happy for very long. You'd also likely need to undersize the frame rail cuts on the printed part and machine them to size, unless you want a really sloppy, rattly gun. But I think my biggest objection is that a plastic 1911 frame would be utter sacrilege! Ol' slabsides deserves steel! :)

On the flip side, a printed frame would be excellent to create a cutaway model for gunsmith training purposes - no need to carve up a perfectly good frame to illustrate the workings of the internals throughout the firing cycle.

Once again I feel compelled to point out that US law, the ATF and indeed the 2nd Amendment is utterly irrelevant to more than 95% of the world's population.. I can print out AR-15 Receivers until i'm blue in the face and the ATF couldn't care less. 8-)

Could hard points be embedded during the print? Wouldn't even need to be metal carbonfibre might help.

Yes! This is precisely the idea I've been pondering - specifically, using plates on either side that have been pre-drilled with the hole locations. With cavities added to either side of the part, the print could be paused near the end of the build, the plates inserted, and the print could then be completed, capping off the top of the receiver and sealing the plates inside.

For the moment, though, I'll try running with just the part 'as-is'. It turns out that the holes are just slightly undersized, and need to be reamed out just a smidge to allow the pins to be inserted.

I wouldn't actually use this to fire anything. I'm sure most everyone here knows better, but just in case you really are thinking about using this receiver...

I'm not familiar with type of plastic being used to create this, but firing a weapon with a plastic receiver that is not of the same quality as the polymer used to create the lower halves of mass produced weapons would be terrible. I can't imagine this would maintain its integrity for very long a
ttached to a fully funtioning upper.

The AR lower receives little stress, and the ABS should hold up just fine for use with .17HMR, .22LR and .223 uppers.

You may be interested to know that orions_hammer has successfully built lowers out of both wood and HDPE: http://www.orions-hammer.com/http://www.orions-hammer.com/

This should work just fine for an AR lower. orions_hammer has made homebuilt AR lowers out of both wood and HDPE (certainly softer than the ABS I used), and both have functioned just fine: http://www.orions-hammer.com/http://www.orions-hammer.com/

exactly what failure do you think is going to occur? worst i can see is that the holes retaining the firing group will fail and allow the hammer to strike without a trigger pull, and the safety on.

next worst is that the stock will snap off during a firing cycle and permit the bolt carrier group to exit the main body of the firearm and impact your shoulder. since it is about an inch across, round, and chamfered, that does not sound likely to cause more than bruising.

According to this article it is both legal and not against policy to post this. So give it a rest.

This article is underhandedly making fun of people like you, those afraid of guns. Good Game.

If a name is not given then a response is not needed.

I'm sure the ATF is much too busy forcing legal firearms dealers to illegally sell rifles to know criminals.


and its not even illegal in the US for christsakes.... All the people whinning about "weapons," dont worry, the big bad ATF wont come over here and mess with your little plastic toys...

Dont you see how "mala prohibita" type laws have warped your mind a little? Are you scared the government is come down on you for prinitng out a piece a plastic? Thats legal in the first place? The proliferation of arms among lawful people can only be a good thing. This country was founded on that
idea. We kicked the damn British the hell outta here for it.

Besides, I think the ATF and Obama's DOJ may be a little busy at the moment, you know, that whole arming mexican drug cartels thing/congressional investigation.... I wouldnt worry to much.

Don't think this is going away:


And you really should try to calm the namecalling down just a bit. Yeah, a gun owner with a bad temper and a big mouth... what could go wrong?

If you could sort of duplicate the shape of the Magpul trigger guard, that would be great. The extra width on the grip end, and that little bit of bulge out makes it a lot easier and more pleasant to heft the rifle with the middle finger of your firing hand during magazine changes. At the same time, that little span also has to be strong enough to take the weight of the rifle.

Yes, the Magpul trigger guard is what I wanted to do originally, but I decided to leave the bottom flat, as I've had mixed results when using breakaway support material on non flat-bottomed parts. (I printed it with the magwell broach axis aligned with the Z-axis)

Ideally, I'd like to do a lower with an integrated grip as well - maybe something like the old Pachmayr 'Vindicator' grips.

im waiting for the upper and other parts so we can print out a full size non functioning AR-15

some of the parts can be functional, just not in a full plastic print

Already been done (the upper, anyway), though not by me: http://m4carbine.net/showpost.php?p=1103272http://m4carbine.net/showpost....

I'm wondering if a printed upper could be made to work - you'd almost certainly need a piston system rather than direct impingement, though!

I wouldn't want to be the one testing a printed upper. :-P

Reprap really needs some support material. That looks awesome.

With the new Mk7 extruder, I'm hoping Reprap/Makerbot will have support material before too long. Ideally soluble support, as trying to clear breakaway support material from the long, small diameter holes in the receiver is quite tedious.

I have been pondering a more Reprap/Makerbot friendly lower as well - there have been several 'built up' lower receiver designs such as the KT-15B (http://www.ktordnance.com/kto/detail/kt15bx.html)http://www.ktordnance.com/kto/... and elmgrove1765's scratchbuilt receiver (http://web.archive.org/web/20071208145613/http://www.geocihttp://web.archive.org/web/200...
http://ties.com/elmgrove1765/project6/project6.html)ties.com/elmgrove1765/project6.... It may be possible to print support-less parts and then assemble them, but the additional joints might make everything too sloppy to work.

Wow! Awesome lower. I'm interested to see what calibers above .22 this can handle.

.223 should be OK.

.308..... maybe.

.50 I'll let someone else try out.

A .223 would probably be okay for a while. However a .308 will not work in any AR-15 lower, metal or otherwise. The round is 2 long and 2 strong. The larger AR-10 is chambered in that caliber but I think the design in plastic would need heavy modification in order to stand up to the punch of a .308. I hunt deer with that round and it doesn't mess around.

You can get Ar15 uppers in pretty much any calibre available. 7.62 NATO is pretty popular, so there's lots of uppers.

50 is a niche market, but there's still a selection of uppers that will drop on an AR15 lower. Neither will work to feed through the magwell, but the lowers bolt on. In theory, the lower doesn't do much so it should work on a bolt action upper, but I would remotely bench test that a whole bunch before I ever held it up to my face.

BTW, the best non-standard AR15 upper I've seen is the AR57.
5.7 has very low recoil and would likely work extremely well with this lower, and they use the magwell as a chute to release spent shells.