Collapsable HK Stock Build

[SwerveAM]

Just finished up with a version to get the reverse stretch up just above 29" overall length. Extension for the A2 is about 5.75" to get there, perfect for those of us who prefer that "extra" length of pull

Also started a version for my HK53 clone (started life as a Vector 93 pistol), which also works on the HK91.

Next up my MPX (stamp due any day now!) and gotta get on that Tavor!

Very cool design

 

[Slim]

Here’s my latest plan. 6” & 3” threaded rods. 1/4”x20. Threaded couplers as inserts in the stock body. Held in place with epoxy, maybe pins if necessary.

The furniture nuts will be between the buttplate and a dual-layer insert (kydex) with clearance to the heads in the rearmost later. Hex bolt holes accessible through the buttplate.

Threaded portion shown extended is ~2.3”. I can stretch that a little bit with a longer lower threaded rod.

Thoughts? I’m not an engineer, so constructive feedback & tips are welcome.

 

[Hawkeye]

So you'd adjust it with a hex key by turning each bolt? That's going to be some slow adjustment! (Not that you'd ever have to adjust it, really, but still.)

 

[Slim]

So you'd adjust it with a hex key by turning each bolt? That's going to be some slow adjustment! (Not that you'd ever have to adjust it, really, but still.)

Yes, yes, and exactly.

I studied some of the 3-gun, trap, and other competition shotgun adjustable buttplate designs. Many of those are adjustable to fit the shape of the shooter, vice clothing, armor, etc. the Magpul PRS stock with the adjustment knobs fitted into slots in the stock is a clever commercial way to do it, but way beyond my ability.

To your point, it’s not just slow, but tedious too because the user will have to adjust both almost simultaneously, or alternate from one to the other, because the buttplate won’t want to flex/angle as it extends/retracts. I’m not sure the spacing between the bolts will permit two tiny IKEA S-wrenches to fit and be turned in parallel, but that would be ideal.

As for how often I’ll extend it? Ideally just once to show you guys here in the forum, so someone can build the next generation on their MP5A2 SBR build! After that, I’ll be a “just in case” thing, like the copies of my Form 1s that I’ve never shown anyone.

 

[Slim]

Been working on tearing down two A2 stocks for this project.

First one is a USA-made clone from HK Parts.net. Clean seam between body and buttpad, but nothing to really grab onto or a gap to pry. Put the buttplate in the vise and put angular pressure on the stock body. I heard a crack and the glue gave way, then it came apart.

 

[Slim]

Second one was a mess!

Nasty looking glue/weld seam. Tried heat, Xacto knife, prying. Eventually I took a saw too it. It was an inter-generational family project, with my grandfathers mitre jig, dad’s workbench, and uncle’s thin handsaw. Took forever sawing through that glue.

HK plastic (I think that’s what this one is) is damn stout. Now trying to figure out how to fix the extension/retraction mechanism in there...

 

[Slim]

Necropost for Halloween

Two years later, still questing for an A2- looking stock that’ll get an MP5 clone SBR to 29”.

I saw this on Instagram today:

 

[yellow97tt6spd]

Funny I just stumbled on this thread all these years later, but both of these designs, are the two that I was talking to my friend about doing for my SP5k PDW. Problem is, with it being a K, I’m more worried about being able to keep folder, and as such, I’m just going to be going to AOW (two handed “pistol”). I’d actually planned to do it on a skeletonized UMP style folding titanium stock. I had figured on getting ultra fine thread 1/4” stainless rod, and running it through threaded tube… for ultimate buttpad placement precision. Figured the only modification from the UMP looking stock I would need, is the lower 3” horizontal tube.

 

[Collector98]

Would anyone happen to have the tubing sizes that were used?

 

[Gcs7th]

Second one was a mess!

Nasty looking glue/weld seam. Tried heat, Xacto knife, prying. Eventually I took a saw too it. It was an inter-generational family project, with my grandfathers mitre jig, dad’s workbench, and uncle’s thin handsaw. Took forever sawing through that glue.

HK plastic (I think that’s what this one is) is damn stout. Now trying to figure out how to fix the extension/retraction mechanism in there...

Slim did you ever make this happen? Or do you have a box of broken HK parts.

Would anyone happen to have the tubing sizes that were used?

Tubing looks to be close to the internal diameter of the stock this looks like a very nice build on the second hand I bet this part could be easily 3d printed to adapt for any size tubing available.

For the OP out there, amazing job. This is on my list still. One day…

 

[Collector98]

Slim did you ever make this happen? Or do you have a box of broken HK parts.

Tubing looks to be close to the internal diameter of the stock this looks like a very nice build on the second hand I bet this part could be easily 3d printed to adapt for any size tubing available.

For the OP out there, amazing job. This is on my list still. One day…

I just bought an AP5 and am hoping to do this in the next few months.

 

[calicojack]

I was going to say why not just buy a collapsible stock. Then I saw how much they cost.
[EDIT] Oh I see, it's an OAL thing.

 

[Vile Shadow]

Got a email from @Atlantic Firearms.com

Looks like they sellin collapsibles for these now

 

[Collector98]

I just got a stock in today and decided to start working on it. I cut the end of it off with a miter saw and got a super straight cut in about 5 seconds. I then took some measurements and am currently printing a test piece. The idea is to screw the buttpad to the "arms" mocked up in Fusion 360, then drill an extra hole in the stock that will just use an extra mp5 pin that goes through a hole in the lower arm to index the length you want. The idea works in my head, but we'll see how it works/fits in real life soon!

 

[smdub]

Does the new stock have the 'rubber bumper' buffer inside? Mine that is one pc molded plastic like that doesn't have a MP5 buffer. Just a metal plate. I think it was from an older G33 w/ the buffered carrier. You don't want to run the gun w/o a buffer.

 

[Collector98]

Does the new stock have the 'rubber bumper' buffer inside? Mine that is one pc molded plastic like that doesn't have a MP5 buffer. Just a metal plate. I think it was from an older G33 w/ the buffered carrier. You don't want to run the gun w/o a buffer.

Mine just has the metal plate on it. I've read conflicting things about the buffer's usefulness on 9mm, but I may just be safe and throw one in anyways.

 

[Collector98]

Final(ish) version. It's 3D printed arms that collapse into the buttstock with varying lengths of adjustment using friction fitting pins to index the length. I only had blue filament, so for the time being, that's how I'll keep it.

 

[Collector98]

Alright, I just decided to paint the pins and it looks awesome! At least to me. It's not perfect, but it locks up tight in the closed position, so I'm very happy with it!

 

[Eastwind]

Alright, I just decided to paint the pins and it looks awesome! At least to me. It's not perfect, but it locks up tight in the closed position, so I'm very happy with it!
View attachment 378435 View attachment 378436

That looks amazing. Would you be interested in doing this as a service for paying customers? Unfortunately OP seems to have disappeared.

 

[Collector98]

That looks amazing. Would you be interested in doing this as a service for paying customers? Unfortunately OP seems to have disappeared.

I'll send you a pm. I remember you asked about it a little bit ago, but I just wanted to tweak it a bit before reaching out.