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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
 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
Reinforced AR-15 Lower Receiver by HaveBlue is licensed under the Creative Commons - Attribution - Share Alike license.
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