[Kaytro]

Id like to see a picture of an mp5 mag being held in an xcr lower mag well without modifications….
I’m not saying it can’t, I’d just like to see it because it doesn’t have the same release mechanism. Not even close. Has someone been sniffing superglue? Or is there something we’re missing?

I was surprised too, But they are both push button release magazines with a notch on the left hand side.

 

[Kaytro]

You mean these things?


Why would you lie on the internet like that?!

Plastic mags are too wide, metal mags are the only type that fit.

 

[Sean K.]

But why? Yeah, they fit...but so what? Just a fun fact?...which is totally fine, just a bit random (and we love random around here so I'm not busting your balls). I do think RA is making a mistake using Glock mags for the 9mm PCC, but it's their mistake to make. I doubt they'd use HK metal MP5 mags and that would make even less sense than the Glock ones.

 

[Kaytro]

But why? Yeah, they fit...but so what? Just a fun fact?...which is totally fine, just a bit random (and we love random around here so I'm not busting your balls). I do think RA is making a mistake using Glock mags for the 9mm PCC, but it's their mistake to make. I doubt they'd use HK metal MP5 mags and that would make even less sense than the Glock ones.

You are right, it was quite random. But! The magwell of the XCR is weird. There is a middle cutout in the lower's magwell, not much, but a noticeable amount. Like a Normal AR15 lower can't fit a mp5 mag. I also maybe emailed RA about it and they said that are testing it.

Tbh, I don't remember the reason why I mentioned it. But I remember that some weird ar lower's were double cut for M3 grease gun mags back in the early 2000s. Maybe the XCR was double cut too.

 

[Sean K.]

You are right, it was quite random. But! The magwell of the XCR is weird. There is a middle cutout in the lower's magwell, not much, but a noticeable amount. Like a Normal AR15 lower can't fit a mp5 mag. I also maybe emailed RA about it and they said that are testing it.

Tbh, I don't remember the reason why I mentioned it. But I remember that some weird ar lower's were double cut for M3 grease gun mags back in the early 2000s. Maybe the XCR was double cut too.

Interesting....Fun fact. 😀 Glad you posted the video....figured you were a bot, lol.

 

[Chowwow]

You are right, it was quite random. But! The magwell of the XCR is weird. There is a middle cutout in the lower's magwell, not much, but a noticeable amount. Like a Normal AR15 lower can't fit a mp5 mag. I also maybe emailed RA about it and they said that are testing it.

Tbh, I don't remember the reason why I mentioned it. But I remember that some weird ar lower's were double cut for M3 grease gun mags back in the early 2000s. Maybe the XCR was double cut too.

Alrighty, so you aren't a bot after all. It's just that with how thin the MP5 mag is, and the huge air gaps that would be left if you put one in the mag well, I would not consider that fitting - just coincidence. Not to sound too snarky, but I can fit various parts of my anatomy into the XCR as well too and vice versa - doesn't mean it was meant for me to do that. If a 9mm conversion for the XCR lower was planned, then using the MP5 mag was a bad idea because now there is no room for a proper mag well adapter to cover those gaps. Then again the Tommy gun as an open gap between the magazine and the chamber, and that thing has been shooting for over 80 years, so maybe there is some precedence for that.

Back on topic, I've been having trouble finding sources for materials. That is to say, I've found bar stock of 7075 aluminum from just two sources on the internet and they are going for over $220 for a 14lbs bar of the stuff. Yikes. With all the waste surface area I might get from machining a 2"x6" foot long stock, I'm considering on optimizing my tool paths so I can probably squeeze two lowers out of every bar stock. I hear that magnesium is getting harder to come by, so that's where the higher than expected prices might be coming from. Good time to design while I wait out the shortage, I guess.

I'm not too enthusiastic about making an extended bolt catch for the lower, considering how tricky it can be to precisely machine small metal parts and how I don't have any forging or heat treating tools. At least prices for 4140 steel isn't bad, but I think I'm better off working off of heat-treated stock and then gun bluing the final product; it's the only way I know of to avoid machining off hardened scales. What I really would rather do, though, is print a plastic spacer to go between the regular length bolt catch and the catch foot and attach them together with an inch long socket head cap screw. Maybe not optimal but most of the stresses the catch experiences are at its head, and a longer SHCS should be rigid enough for the shooter's hand to manipulate the foot.

 

[Sean K.]

Alrighty, so you aren't a bot after all. It's just that with how thin the MP5 mag is, and the huge air gaps that would be left if you put one in the mag well, I would not consider that fitting - just coincidence. Not to sound too snarky, but I can fit various parts of my anatomy into the XCR as well too and vice versa - doesn't mean it was meant for me to do that. If a 9mm conversion for the XCR lower was planned, then using the MP5 mag was a bad idea because now there is no room for a proper mag well adapter to cover those gaps. Then again the Tommy gun as an open gap between the magazine and the chamber, and that thing has been shooting for over 80 years, so maybe there is some precedence for that.

Back on topic, I've been having trouble finding sources for materials. That is to say, I've found bar stock of 7075 aluminum from just two sources on the internet and they are going for over $220 for a 14lbs bar of the stuff. Yikes. With all the waste surface area I might get from machining a 2"x6" foot long stock, I'm considering on optimizing my tool paths so I can probably squeeze two lowers out of every bar stock. I hear that magnesium is getting harder to come by, so that's where the higher than expected prices might be coming from. Good time to design while I wait out the shortage, I guess.

I'm not too enthusiastic about making an extended bolt catch for the lower, considering how tricky it can be to precisely machine small metal parts and how I don't have any forging or heat treating tools. At least prices for 4140 steel isn't bad, but I think I'm better off working off of heat-treated stock and then gun bluing the final product; it's the only way I know of to avoid machining off hardened scales. What I really would rather do, though, is print a plastic spacer to go between the regular length bolt catch and the catch foot and attach them together with an inch long socket head cap screw. Maybe not optimal but most of the stresses the catch experiences are at its head, and a longer SHCS should be rigid enough for the shooter's hand to manipulate the foot.

7075 is not cheap. I'd imagine you'd need to buy huge ingots in quantity for it to get reasonable. With metals pricing over the last few years, and rampant inflation....I doubt you're going to see an abundance anyway.

You could send the 4140 out to a heat treat company. I've had some parts heat treated here by 2 companies (Phoenix Heat Treat was one...can't remember the other)....they charge a reasonable set up fee and then by the pound.

 

[SheffieldTactical]

They do fit and feed! No shit! I’m interested lol

 

[Chowwow]

They do fit and feed! No shit! I’m interested lol

No shit? Does the mag rock or anything at all? Are the air gaps an issue? Maybe we better move this part of the conversation to the XCR-P thread. Finding magazine conversions for 9mm aren't a big issue, the big issue is finding a barrel and bolt head combination that will actually fire and cycle the thing. The thing I'm wondering about is if the lugs on the existing bolt head design can properly strip rounds off a Glock or CZ Scorpion mag. If so, we can just open up a 5.56 bolt a bit for 9mm, if not then we need to design a new bolt. Machining a new bolt would be hard too; I'd definitely need a 5-axis CNC machine of some kind that's very rigid out of some really strong alloy steel stock.

I'm about 40-50% done with designing on my current project, but I feel like I can jump onto this thing in earnest a little before that.

 

[Kaytro]

No shit? Does the mag rock or anything at all?

It is a less wobbly fit than a real MP5s magwell.

 

[Chowwow]

I was wondering what this pocket towards the rear of the lower (indicated in red, behind the FCG) was for? I suspect it's to provide space for the auto sear but I'm not interested in implementing that feature nor would I know how an XCR auto sear would work. Does that empty space serve any other purpose? The area needs to be low enough for the bolt to travel over it but otherwise, it's a lot of empty space. Maybe it's a channel for air inside the the receiver to rush out when the bolt moves back and forth at high speeds? Depending on how accurate I need to make this pocket, it could affect how I machine the lower with a 3-Axis.

 

[Chowwow]

This last week I've been brain storming possible ways to make an alternative XCR lower with a focus of accepting AK magazines. However, I can't seem to figure out a way to integrate the Mag release of the AK into the mag well area of the XCR, because the mag release lever is in the way of the bolt release of the XCR.

An extended lever shown for detail.

I could adopt the bolt catch system in the AR-15 instead and save a ton of space, if people wouldn't mind but I sense doing so would erode the point of having an AK mag lower for the XCR. Alternatively, I could adopt a different system to hold the AK mag, but I haven't seen other designs out there that would work. Ideas?

 

[roflcopters]

get a bren2 7.62 mag
It's 0.1" wider and longer than STANAG, but shares mag release and LRBHO. Also, some of the front of the magwell will need to be removed

 

[Chowwow]

get a bren2 7.62 mag
It's 0.1" wider and longer than STANAG, but shares mag release and LRBHO. Also, some of the front of the magwell will need to be removed

I see, so the Bren 2 mags are more conducive to the XCR form factor. Are they really much better than the ones Robinson was recommending? I thought Sean was hoping to get regular AK mags to work on this platform, but I can't seem to figure out how to get them working at the moment.

 

[Sean K.]

I see, so the Bren 2 mags are more conducive to the XCR form factor. Are they really much better than the ones Robinson was recommending? I thought Sean was hoping to get regular AK mags to work on this platform, but I can't seem to figure out how to get them working at the moment.

It's been done with custom machining to a factory lower for regular AK mags.

The Bren 2 mags have been discussed and would be a good option....if it weren't for the problems CZ is having with their X39 Brens. I'm not sure they are going to keep making them. Would suck to develop around a mag that gets discontinued.

 

[Chowwow]

It's been done with custom machining to a factory lower for regular AK mags.

Wow, I've haven't heard of those yet. So how do the mags rock in and rock out, did the lug just tab into the back of the mag well with no mechanism holding it? My issue with the design so far is trying to get an AK mag release to work with the XCR's bolt release in the way while maintaining position with the bolt and feed ramp.

I just read an old post where Terra said that the Upper would have to be redesigned to fit AK mags. That sounds unreasonable to me but who am I to discredit someone who worked so close to the kitchen at the time?

 

[roflcopters]

I see, so the Bren 2 mags are more conducive to the XCR form factor. Are they really much better than the ones Robinson was recommending? I thought Sean was hoping to get regular AK mags to work on this platform, but I can't seem to figure out how to get them working at the moment.

Correct, as mentioned above availability might be an issue

 

[Sean K.]

Wow, I've haven't heard of those yet. So how do the mags rock in and rock out, did the lug just tab into the back of the mag well with no mechanism holding it? My issue with the design so far is trying to get an AK mag release to work with the XCR's bolt release in the way while maintaining position with the bolt and feed ramp.

I just read an old post where Terra said that the Upper would have to be redesigned to fit AK mags. That sounds unreasonable to me but who am I to discredit someone who worked so close to the kitchen at the time?

Not sure how the guy did it, but it worked (he had videos) and he didn't redesign the upper. Personally, I'd say stick to what you're doing....getting the AR triggers you want to work....not focusing on an AK lower. One thing at a time.

 

[Chowwow]

Not sure how the guy did it, but it worked (he had videos) and he didn't redesign the upper. Personally, I'd say stick to what you're doing....getting the AR triggers you want to work....not focusing on an AK lower. One thing at a time.

Yeah I get it, I'm taking care to not pull too many strings at once. I'm still curious as to how someone managed to get an AK mag to work like that though, are the videos on Youtube or a different site?

 

[Sean K.]

Yeah I get it, I'm taking care to not pull too many strings at once. I'm still curious as to how someone managed to get an AK mag to work like that though, are the videos on Youtube or a different site?

They are here (but hosted on YT IIRC)....but the search engine sux here. When I get time, I'll try and look for it.