Poll for the next XCR Lower project!

Sean K. said:

Have you decided to go with polymer mags only to avoid having to sleeve the internals of the lower?

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You know what? I've decided to keep it simple and stick to making the AK-XCR lower all aluminum and just advise that users use polymer mags with it. It's just that making a steel sleeve that press-fits into the receiver appears to be technically out of my reach in the near and mid-term; with the type of milling machines available to me, I don't see how I can build a steel sleeve accurately and consistently enough to get good results. Steel extrusions would be the best part for this kind of job but I don't have the tech to build my own steel extruder or the money and space to order thousands from a supplier. Instead, if/when I design the swappable magwell lower, I will make the entire AK magwell section out of steel that will pin to the other half of the lower and that assembled lower will gladly accept steel mags.

In other news, I was able to buy the mag release and associated hardware for the CMMG MK47 from CMMG's customer service. Cost me $38 and some change. This'll help a lot with this design job, though how I intend to include one for each of the production lowers isn't something I'm thinking about right now. I suppose I can copy the design of this mag release or something similar when the time comes. Still, it's nice to know that we can rely on a major company to be producing a critical part for this lower.

First print of the AK-XCR lower fresh off the printer. This lower uses the XCR trigger group.


Here it is without supports. This one was printed with tree supports to save on filament but had compromises on accuracy. Still, it's good enough for a first look.


Here it is with the Standard upper. The magwell is wider than that on the standard XCR lower. That's necessary because there is so little meat on the sides of the magwell on the AK-XCR. I'll later expand the magwell to be flush with the outer contour of the lower with the hammer pin side.


Before the lower is installed, I checked how far an AK mag can enter into the upper and I found that it doesn't go that far inside; not as deep as an AR-15 magazine can.


Here is the lower with the Palm USA mag. The mag is held at a bit of an angle; we can level it off by trimming off the wall behind the mag.


Bottom view of the magwell of the lower. I noticed that abran007's lower mounted the magazine closer to the front of the rifle, while mine has about 7/8" of material between the magwell and the front of the lower. Considering that his lower works and mine is a prototype, I'm leaning on his design.


Both mags have a bit of a beak that is typically used to secure the front lip of the magwell. The magazine does not go far enough inside the upper, so the magazine just sits there, forced back by this beak. I might have a shoulder there to catch that beak in the next design, or a clearance to get around it.


Here is a fitting with the CMMG mag latch. There is not enough space for the mag latch now, but I can see where I can machine material off to fit it.


So my main focus here is getting the magazine to fit in a functional manner, followed by adapting the CMMG mag latch to work with this system. I'm assuming the trigger group and pistol grip et al are still functional as they are untouched from my old work.

In regards to the magazine, I think I need to shift the magazine forward and level it so that hopefully, the bolt can strip and load rounds reliably. I can't raise the mag higher but abran007's lower seemed to work fine this way. The walls of the magwell are really thin and installing a steel sleeve (with a magwell flare) is going to be a challenge. I'm focusing on designing the lower as one piece for now, but I'll keep that requirement at the back of my mind until I can get the magwell to function right.

With this much space, I feel like the CMMG mag latch can be used in both AK-XCR lower models (for the XCR trigger and the AR trigger). Looking at the prototype lower, there doesn't seem to be enough space for the CMMG mag latch. Looking at the Mk47, I noticed that the trigger guard is much narrower than that a typical AR (and the XCR) and that seems to provide a lot of the space for the mag latch. I may have to follow suit and shorten the trigger guard on the AR-XCR to make room; I was hoping the XCR lower was long enough to avoid that but we need to get this system working. Pushing the magwell forward almost a 1/4" aught to help a lot with that though.




I'll take a dremmel to the first print and work around the material to see at what point I can make the magazine and mag latch to fit just right. Those points will help with the next set of designs.

Flskunkape said:

@Chowwow the prototype lower looks great, so something else that abran007 did on his was extend the right side of the receiver down similar in shape to a fal receiver, I'm not sure it would make the magazine set anymore secure like as in left to right wiggle, but it sure looked cool. Either way seeing an xcr upper with a lower that takes ak banana mags is awsome!

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I remember seeing that and figured it was temporary reinforcement until he had the design refined. I'm trying to keep the design as light and refined looking as the CMMG MK47 rifle. I don't think it's going to be necessary in my design though it kind of bothers me that the magwell is so thin while still needing some thickness for that steel sleeve. I'm hoping it doesn't look to ballooned out in the end; otherwise I'm enjoying how small and light this model is.

I'm examining the CMMG mag release and I realize that it's a CNC machined part; so I should be able to reverse-engineer it and produce a part compatible with the MK47 family of rifles. It's made of some kind of aluminum alloy, which makes it light and easy to use on polymer mags, but might wear out against steel mags. The final one I make for the AK-XCR might end up as some kind of steel alloy. I was attracted to this mag release design because it was ambi, long enough for a AR-pattern lower, and made by a major company. I was hoping I could use generic AK mag releases for this design but had little guarantee they would work on the taller lower that uses the AR trigger group.


The nifty thing about the CMMG set is that the pins are threaded, so they are easier to install onto the lower. I would like to use these same pins and spring on the AK-XCR, but that would depend if I can get them in bulk from somewhere like McMaster-Carr.

I made a 3D model of the CMMG mag release in CAD, accurate to +/-.005". With this, I should be able to make accurate copies of the mag release; a 3-axis CNC machine should easily cut one out. The original one is made of some kind of aluminum; the release for the production AK-XCRs should be out of some kind of steel alloy to work with the steel AK mags.


Using my available CAD assets, I made an assembly to simulate the functional parts of the AK-XCR so far:


The magazine appears to be placed correctly based on current experience. The CMMG mag release is positioned so the angled tip is flat against the tab of the magazine.


As we can tell from the CAD model, the pivoting pin that the mag release relies on needs to be located deeper inside the lower. This means the trigger guard and trigger hole needs to be shortened to make way for the mag release and pin hole, like the ones on the CMMG M47 is. It's not a big deal for me, but making the trigger hole smaller could make it harder for gloved fingers to reach into the trigger. I hoped the XCR was long enough to avoid doing that but it looks necessary. Abran007's lower did not look like it needed the trigger hole to be smaller, indeed it's the normal size. Looks like the custom mag release he was using was a much thinner profile than the CMMG release. Keep in mind the current design uses the XCR trigger and is low profile; the AK-XCR w/ AR FCG will be as tall as the AR-XCR and will have a bit more meat from height for the mag release.

This creates the first decision branch for this project. I can either keep using the CMMG mag release knowing I would have to shrink down the trigger guard, or build a custom low profile mag release that is similar to abran007's so as to maintain the shape of the XCR trigger hole. The custom mag release might not look as nice as the CMMG release and will end up being a proprietary part from me, and no matter what I do it would be tricky to be able to provide all of these small parts when extras are needed.