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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.
 

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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.
 

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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.
 

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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.
 

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Discussion Starter · #26 ·
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.
 

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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.
 

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Discussion Starter · #29 ·
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.
 

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Discussion Starter · #31 · (Edited)
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.
Trigger Air gun Gun accessory Automotive exterior Carbon
 
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