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Posted: 1/6/2026 9:31:42 PM EDT
| I was considering getting a 10mm pistol. I want a 8-10inch barrel. What are the opinions of the CMMG pistol in 10mm here? Is there another brand in 10mm you would recommend. Thanks. |
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There is a recent thread about someone having bulged cases in their CMMG pistol https://www.ar15.com/forums/handguns/Underwood-10mm-200gr-xtp-cases-bulging/20-218226/ |
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The CMMG RDB system kills ejector springs in 9mm / 40SW and 10mm. That said, CMMG finally released the FE (Fixed Ejector) versions in February of 2025 but only for 9mm and 45. They initially said they would support the old and new but the old ones have disappeared off their site minus some parts. It is obvious the new is superior. https://c3junkie.com/?page_id=221 That said, CMMG did post on Reddit that they were going to come out with a 10mm FE version. So I wouldn't get a 10mm CMMG now. Wait till that comes out. Back to regarding the chamber, CMMG made it clear when they released the 10mm that the chambers were a little big and that it wasn't anything a sizing die would fix if you reload (their words not mine). I don't run 10mm but I have a friend that does. He also got a custom 10mm barrel from Macon Armory that he uses in his 10mm Banshee with a fully supported chamber. His also kills ejector springs like I mentioned. |
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Originally Posted By DOG556: So CMMG has non supported chambers in 10mm??? From the picture I would not say that CMMG has unsupported chambers. But, I do not have one to examine. What it appears is that there isn’t enough weight in the bolt and buffer combination. The bolt is opening while the pressure is still high. I’m using a PCC AR with a 16” barrel, and I’m using a 35oz combo. For an 8-10” barrel, I would expect a weight of about 32oz(combined weight). This weight would be for a 180gr bullet at about 1350fps. Which should be about what one would expect from a 8-10” barrel. |
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Originally Posted By amphibian: The CMMG RDB system kills ejector springs in 9mm / 40SW and 10mm. That said, CMMG finally released the FE (Fixed Ejector) versions in February of 2025 but only for 9mm and 45. They initially said they would support the old and new but the old ones have disappeared off their site minus some parts. It is obvious the new is superior. https://c3junkie.com/?page_id=221 That said, CMMG did post on Reddit that they were going to come out with a 10mm FE version. So I wouldn't get a 10mm CMMG now. Wait till that comes out. Back to regarding the chamber, CMMG made it clear when they released the 10mm that the chambers were a little big and that it wasn't anything a sizing die would fix if you reload (their words not mine). I don't run 10mm but I have a friend that does. He also got a custom 10mm barrel from Macon Armory that he uses in his 10mm Banshee with a fully supported chamber. His also kills ejector springs like I mentioned. It is possible to fix....the CMMG chamber mouth is belled and it was done to solve feed issues, Not much can be done with the existing barrel (part of problem is the extension as well) but I have made several that correct the issue. |
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Does the 5.7x28 CMMG also kill ejector springs? Or are these fixed ejectors??? Originally Posted By amphibian: The CMMG RDB system kills ejector springs in 9mm / 40SW and 10mm. That said, CMMG finally released the FE (Fixed Ejector) versions in February of 2025 but only for 9mm and 45. They initially said they would support the old and new but the old ones have disappeared off their site minus some parts. It is obvious the new is superior. https://c3junkie.com/?page_id=221 That said, CMMG did post on Reddit that they were going to come out with a 10mm FE version. So I wouldn't get a 10mm CMMG now. Wait till that comes out. Back to regarding the chamber, CMMG made it clear when they released the 10mm that the chambers were a little big and that it wasn't anything a sizing die would fix if you reload (their words not mine). I don't run 10mm but I have a friend that does. He also got a custom 10mm barrel from Macon Armory that he uses in his 10mm Banshee with a fully supported chamber. His also kills ejector springs like I mentioned. |
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Originally Posted By DOG556: Does the 5.7x28 CMMG also kill ejector springs? Or are these fixed ejectors??? |
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what actually causes the ejector springs to fail? i would think a pistol cartridge being lighter would be easier to eject than a rifle cartridge. tho i know dual ejectors seem to be a good upgrade in AR10 builds EDIT: nevermind i saw your post about it on your website. that seems like such an odd issue. maybe the bolts rotation while theres still too much chamber pressure? i would think that if a cartridge is seated in the bolt and the ejector is already compressed, how is it damging the spring? Mad, do you ever see these issues with your DI setups? |
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Originally Posted By Kingfish999: what actually causes the ejector springs to fail? i would think a pistol cartridge being lighter would be easier to eject than a rifle cartridge. tho i know dual ejectors seem to be a good upgrade in AR10 builds EDIT: nevermind i saw your post about it on your website. that seems like such an odd issue. maybe the bolts rotation while theres still too much chamber pressure? i would think that if a cartridge is seated in the bolt and the ejector is already compressed, how is it damging the spring? Mad, do you ever see these issues with your DI setups? Nope...... What causes spring failure is the fact that the pin ejector pushes the case off the face of the bolt and holds it against the extractor as the round is never actually locked or headspaced. When it fires...the case compresses the spring faster than it is capable of handling and till is seats on the bolt face and cycles. Just my guess.... 
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Bulged cases could be the result of 2 things. An unsupported chamber, OR the action is opening too soon, while there is still too much pressure in the system. This could be the result of not having enough weight in the bolt and buffer combo. Or, by not having enough recoil spring. I just put together a 10mm carbine, and I’m using around 35oz with a SpringCo Orange spring. Pretty much the strongest they make. I think I have finally got it right, or at least close. So, without examining the gun, it’s hard to say. The weight issue is probably fixable . The bulged chamber issue, if it exists, may not be easily fixed. Bolt/buffer weight combo depends on bullet weight and velocity. With my 16” barrel, I’m getting about 1600fps with a 180gr bullet. Since I don’t know what barrel length the CMMG pistol used, assume an 8” barrel. Which should give a velocity around 1350fps. It’s doubtful that it would exceed that. That would require a moving weight of about 32oz, which would be relatively easy to do. |
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