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NERF Meaker Mk 18 Assault Pistol

by MeakerVI Nov 15, 2018
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I believe that the MK18's structural problems have to do with the ratio between plunger and barrel volume.
After the dart leaves the barrel (that is: immediately, if the spring is strong enough) there's nothing that counters the force of the spring.
And then the plunger slams, undamped for a significant part of its way, into the front part and sooner or later something breaks.
Therefore I'd suggest not to use hard springs like the K25 (which is probably illegal in many countries e.g. in Europe) but softer springs.
In that case the R3 variant should be strong enough, or what are your experiences?

On the other hand, I've never been able to successfully shoot 3 darts at once. For a reason unknown to me, one dart will always fly first, and most of the air pushes out of that free barrel instead of launching the other darts... is that because my spring is too soft?

And then the plunger slams, undamped for a significant part of its way, into the front part and sooner or later something breaks.
Therefore I'd suggest not to use hard springs like the K25 (which is probably illegal in many countries e.g. in Europe) but softer springs.

A softer spring would save wear & tear, yes, but would also lead to other issues as you've noticed: springloads light enough to avoid structural damage will also have a hard time with anything other than single shot. Padding the plunger head is probably possible and might be a good idea, but IDK if alone that would save it.

As to legality; I've never clocked a single shell above 110 FPS, which is a bit under double stock nerf but only ~10-20% more than stock adventure force/zuru/dart zone. Barrel length is tremendously important to dart velocity: making a much longer shell out of brass and an adapter, I've hit 180 FPS without changing anything other than the shell. If you're making modifications, it's also possible your seal isn't quite right or the plunger volume is just enough less to be significant. I've noticed that this is about the ideal size, and a hair less was anemic.

I've had some success by tying the plunger head to the back of the frame such that it can't quite hit the front at full extension.

Thank you - I imagined you'd have tested this a lot.

110 FPS might be just too much for Germany; if I'm not mistaken the limit there is 0.5 Joule.
Here in Italy the limit is at about 160 fps / 1 Joule, so I should be safe with a K25, at least with a short barrel.

The thing about barrel length sounds about right. For optimum efficiency you'll need a barrel that has about the volume of the plunger (but always a little less) to transfer most of the spring's energy to the projectile - if the barrel is too short, the air that escapes after it won't be able to push the projectile forward. Or could even interfere with it's trajectory.

I've experimented with tolerances between seal and plunger head, and yes, already small differences can change the behavior completely. Even printing the plunger head with different material or extrusion temperature changed attrition/sealing of the plunger.
If you use a weak spring, the seal can't be too tight or attrition gets too big for the spring. Even if that seal would work perfectly with a stronger spring.

I personally am not interested in high power (I got kids) and triple shots, but I like your design. So I made a Nightfinder-Style blaster out of your MK18, with a smaller plunger volume (still too big to be efficient I think, but it works) and an internal front-loaded barrel - and no shell support. I'll try to post the remake this weekend. It's just an upgrade, frame and handle are the same, only the plunger rod and the internal parts are different.

Thanks again!

Any chance you'll offer a metric variant of the Pistol? I really like the design and am keen to print it - but sourcing the material in Europe is really difficult :(

Finding material for the R3 is no trouble at all. I made one, and used wood screws from a set (don't remember which ones) clipped to the correct length. I had to redesign the piston to fit my spring, pipe (32mm OD/28mm ID PVC drain pipe) and O-ring. I can post them as a thing, if you want.

I also made an R4 with M4 threaded rod + nuts. The slots for the nuts are a little too wide, but IMO that's the least annoying thing about the R4. I've improved it a little.. give me a few day and I'll post it - unless Meaker announces a new version, in that case I'll happily remix that one.

Drywall screws shouldn't change much no matter where you are; there are just certain physical constraints to the materials involved that make them work at a certain size.

Tubing might be an issue. @walterw if you've got localizations go ahead and post them as remixes. The EsperMk18 is the spiritual successor to this ATM, hoping to revisit someday with a v2 take on the whole thing.

Is it just me, or does the FRAME part for the Break Action not really fit the rail? It slides around all over the place. Is there anything I need to do in order to lock it?

It’s mostly to lock the coupler from de-coupling and act as a hinge for the break action. You need to bolt through it and the teeth in the pic rail on the front to lock it in place.

Hi again, I'm using a 5,5 inch K25 spring, and the triggerpull is abnormally heavy, is that normal?

Trigger pull shouldn't change based on spring weight, check that everything is moving freely and the chamfer is going the right way. If everything seems fine, try filing down the catch surfaces. I have had issues with a few having abnomally high trigger weight sometimes, but I haven't been able to discern what causes it in some builds but not others.

I'm assuming that the flat part of the catch is the part that should hold back the plunger, not the slanted part.

Yep! Sorry about the late reply, thingiverse isn't sending me messages for some reason.

I printed an R3 at first, and then I realized I should have printed an R4. Oh well that just means I need to pick up some threaded rod. One problem with the R4 that I noticed is that the top rod's nuts both have spots for the nut to be captive so the top wouldn't be able to get tight easily.

I also noticed that the R4 zip doesn't have a catch in it, the R3 has a catch but it's worse than the one in the legacy files.

R3 can be fine, R4 uses the rod but I've had failures with it too now. Current recommendation is to print in a different material or use tree supports and print an R3 at an off-angle.

I'll look at the catches, I didn't remember messing with those that much.

I'd suggest to make two of the "nut nests" captive/tighter (the one inside the grip and the upper front one), and the other two round and flat-bottomed and big enough so that you can tighten the nuts with a socket wrench. There should be enough space for theat (at least for metric M4 nuts).

I couldn't find a fitting spring in Europe - do you know if any "tune your original nerf blaster"-springs fits into this pistol?

Any 5.5"/140mm long nerf spring with an ID big enough to go over the plunger rod (off hand, 3/4"/~19mm OD) should work. The K25 I'm using should have about 11kg draw so standard stock or upgrade springs shouldn't be an issue for it. The bigger stuff you need to replace the internals for might cause problems, but I haven't done much experimentation as the system works reasonably well with the setup it has.

Regarding your suggestions, I'm looking at doing a total rework of several parts to increase plunger volume and frame strength.

I'm looking to build a few low-velocity rival pistols for some younger nerfers. Any chance of a rival barrel with hop-up here?

I've tried a few HIR barrels, but none have worked well. I've provided the solid bodies for the barrels so if you'd like to experiment I'd love to add a HIR shell to the arsenal.

I've also heard hotshocks/bigshocks can fire HIRs basically stock (pull the dart pegs, probably AR's), if that's your objective.

Doesn’t work well in my Hotshock. I’ll play around with some brass/pvc/cpvc/furniture pipe and see what I can find.

is the bottom rail just cosmetic, or does stuff attach to it?

It's designed to be picitinny, so in theory pic-attachments can attach to it. I need to touch up and release my flashlight holder so you can just print that, which is the most reasonable thing that'd go there in a real NERF war.

The break-action attachment also uses it to stabilize itself.

is the plunger tube pvc or is it also 3d printed. and if so, what size? and is there a certain o ring that should go on the head or just find one that fits? and are all the screws the same size. also what is the size of the pins for everything?

Plunger tube is meant to be 1" 200 PSI/Class-200 "Thinwall" PVC. You can try 1" Schedule 40 or other +/-1.315" OD tubes, but expect diminished results.

Dash-212 or 213 orings should work but I've had mixed results depending on supplier.

All screws are #6; on R1 and 2 the holes are chamfered for bugle or flat head but on R3 they're flat for button heads. 1" and 1/2" long; if you used #6 drywall screws 1-1/2" would be plenty long for everything and can be cut down.

Would you mind uploading the handle in STEP or something? I would REALLY like to borrow that lever design for a project. (I'll make one of these too)

The whole project has a few step (or other solid body, thought it was step though) files exported at various revisions in the downloads.

would a mega dart shell be a possibility?

I see I did not reply; but there should be a mega shell in the files somewhere. It's possible if not, but I haven't tested it much and the barrel wasn't dialed in.

what scrow size is needed ? m3?

#6 drywall screws. Drywall is drywall and needs to be a certain thickness and mass to work; so if you have it in your region your drywall screws should be the same and should work. If not, that should be m3.5.

cool. can i have the sizes of the non 3d printed pieces in mm or cm please?

The exact sizes of the required parts are given in inches because the parts are US-specification parts (1” PVC is a nominal-size, not a dimension). I’ll need to design a new set of parts based on tubing/pipe available in your region.

Ok. don't know the exact size that my stores have. i'll let you know after i go to the hardware store next. thanks

The STLs are opening in Cura. But the are not too size. I had to scale them up to 2000% to even see them. I don't know why it is happening.

Files are in inches, Cura expects mm. Scale up 2450%.

In the future I’ll try to set the file up for correct import, I’m still working on a few barrel-related things and would like to keep it in inches until I get them sorted out.