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MoRivera

CMMG Guard bolt/barrel in Mil-Spec receivers?

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Hello All,

 

Was wondering if someone could help me on this.  I'm interested in getting a CMMG Guard bolt and barrel, but wondering if they will fit and function in a regular Mil-Spec upper and lower (with a Stern Defense Lock mag adapter in my case).  On the CMMG site, it mentions a flared ejection port, but how exactly and wondering if that's something I can do myself.  Or does it really need the proprietary CMMG MkG upper and lower receiver.  Would be much easier for me if I could just install into one of the receivers I have handy.  Thanks in advance.

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Mil spec upper works fine. I don’t know about the lower. I know Colt style lowers hold the magazine too high to be used with a Guard bolt without modification. 

If you end up needing to enlarge the ejection port, that’s easily done. 

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Cool, yeah I can see how the ejection port could be enlarged just by grinding back the 'lip' areas a bit but still be covered by the trap door if need be.  Would just need to figure out how to protect the bare aluminum.  Thanks.

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Upper:

 

I've done it twice. To meet CMMG spec you actually need to grind quite a bit in the front half of the upper. The easiest way is just to buy a .458 spec upper and use as-is. They're not expensive. 

 

Like this one at Primary Arms

 

Or if you want a billet one, the NFA/Spartan from Joe Bob's have .458 spec ejection ports on them. That's what I started with and worked fine. 

 

On the lower: Colt mags will require modification to the height of the 'ears' on the feed ramp and likely the mag catch window on the mags themselves or the mag catch (better option). I was going to mess with it, but it was easier just to get an NFA Glock lower. I did still have to mod/file the mag catch (mags sit too high otherwise) and rounded the nose on Glock followers (not necessary), but it was pretty easy. 

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Sweet, thanks much!  The lower is a bigger concern because I don't really have much leeway, being a CT resident and having to go with my preban lower and Stern Defense mag adapter in order to have a 'legally' buttstocked carbine.

Edited by MoRivera

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I know the capacity is lower, but you might look at an Endo Mag setup. Slap a Guard-based upper on your existing lower and go. 

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4 hours ago, Acer2428 said:

I know the capacity is lower, but you might look at an Endo Mag setup. Slap a Guard-based upper on your existing lower and go. 

The Stern Defense adapter that I use works great with Glock mags, completely reliable and I get last round lockback as well. Since it inserts from the bottom there might be a little leeway as to how low I can get where the mag seats, but not much since it still secures in place mainly by the lower's mag catch.  Frankly if I could I probably would have just bought a complete CMMG Guard carbine, but again in CT we're not allowed to buy or build AR-platform rifles (with an actual buttstock vs. arm brace) that are post-'94.

 

In all likelihood I'll probably not bother because of the whole lower issue, but was just seeing what needed to be done.

Edited by MoRivera

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 The stern adapter is set up for glock mags with a blow back bolt. You will most likely have to lower the mag with the guard bolt or it will drag on the mag. The easiest way to do the is to take material off of the top side of the AR mag catch so the entire block sits lower.in the magwell. 

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33 minutes ago, bwikel said:

 The stern adapter is set up for glock mags with a blow back bolt. You will most likely have to lower the mag with the guard bolt or it will drag on the mag. The easiest way to do the is to take material off of the top side of the AR mag catch so the entire block sits lower.in the magwell. 

Yeah, that's what it was looking like.  But also......from what I can tell the CMMG ejector is inside the actual bolt, whereas with the Stern adapter (like others) the ejector is on the adapter itself. 

Edited by MoRivera

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You will have to pull the ejector for sure, or chop it off. 

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Yep. Would 100% not do another franken-guard build. Would just buy it and be done... if you had to. IMO it's not enough of a step up to be worth all the trouble, especially with the recent development in short-stroke kits. You'll also then have to deal with or have your barrel reamed b/c CMMG cut them SUPER short.... like shorter than my CZ short. 

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16 minutes ago, Acer2428 said:

Yep. Would 100% not do another franken-guard build. Would just buy it and be done... if you had to. IMO it's not enough of a step up to be worth all the trouble, especially with the recent development in short-stroke kits. You'll also then have to deal with or have your barrel reamed b/c CMMG cut them SUPER short.... like shorter than my CZ short. 

Agreed.  Again, I'm a CT resident so I have that hurdle of not being able to legally buy a CMMG lower and/or configure it as a rifle.

 

I like how my regular blow-back PCC is running now, and really, if I want less recoil I'll have to start hand loading.  But it was good to ask before diving into a build that might have more problems and cost more $$ than what it's worth.  Thanks everyone.

Edited by MoRivera

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2 minutes ago, MoRivera said:

Agreed.  Again, I'm a CT resident so I have that hurdle of not being able to legally buy a CMMG lower and/or configure it as a rifle.

 

I like how my regular blow-back PCC is running now, and really, if I want less recoil I'll have to start hand loading.  But it was good to ask before diving into a build that might have more problems and cost more $$ than what it's worth.  Thanks everyone.

 

Yep, I get it. Was just trying to save you some headache. 

 

Take the money and time you would have spent on it and start rolling your own. MASSIVE improvement. Hell, it's just more enjoyable. Some 147's and a fast burning powder and away you go.

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If I didn't live in this state I would have bought a Guard or Sig MPX by now....or both, with how much a pre-ban lower costs!

Edited by MoRivera

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