SIGforum
An interesting new roller delayed blowback 9mm coming: Springfield Kuna.
February 08, 2026, 05:33 AM
92fstechAn interesting new roller delayed blowback 9mm coming: Springfield Kuna.
quote:
Originally posted by IndianaBoy:
quote:
Originally posted by Dwill104:
. Plus, the FE bolt also has a revised angle on the bolt lugs that’s supposed to be better.
Obligatory photo.
I have a legacy ejector bolt with the updated 50 degree bolt angles. You can tell which bolt you have. The newer ones are marked with an engraved numerical 50. It does seem to shoot a little softer. But I never had any trouble with the original bolt.
The FE conversion was a little too salty for my taste, but I understand why some have opted to go that route.
Despite several thousand rounds on both bolts, I haven't had an ejector spring fail yet. Just lucky perhaps.
I love having the same manual of arms of my 5.56 ARs. It just feels natural. So smooth with 147gr subsonic ammo.
I do think the CMMG radially delay system is the sleeper PCC that doesn't get as much attention as it should.
Really nice being able to have one SBR lower that can do 9mm with endomags and any of the other regular small frame AR calibers.
I got the revised 50 degree bolt from CMMG a year or two ago. And the carrier weight kit a year or two before that. It still eats ejector springs. On the upside, I now have a spare bolt that lives in the Magpul MIAD grip on the gun that's pretty easy to swap out in the field if they start dribbling out, and then I go home and swap the ejector spring again. I keep spares on hand so it's no big deal, just a minor annoyance. I still love the gun and am actually considering ordering an 8" bolt/barrel combo to put together an SBR now that the tax stamp is free.
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Any comments made by this poster are my own and do not reflect the views or opinions of my employer.
February 08, 2026, 07:50 AM
Dwill104I didn’t know CMMG sold the 50 degree spring ejector bolt alone. I seem to remember looking on their web page, but didn’t see one listed.
February 08, 2026, 02:30 PM
Mars_AttacksMine has the 50 on the bolt. I have ordered the FE kit as well, but am going to get the 5" barrel and shroud a suppressor.
____________________________
Eeewwww, don't touch it!
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February 08, 2026, 04:16 PM
92fstechquote:
Originally posted by Dwill104:
I didn’t know CMMG sold the 50 degree spring ejector bolt alone. I seem to remember looking on their web page, but didn’t see one listed.
I didn't buy it. I contacted support and explained the issue I was having and they sent me one for free. That's also how I got the weight kit for the bolt carrier, and my first batch of ejector springs. Their customer service is awesome.
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Any comments made by this poster are my own and do not reflect the views or opinions of my employer.
February 08, 2026, 09:30 PM
IndianaBoyquote:
Originally posted by 92fstech:
quote:
Originally posted by IndianaBoy:
quote:
Originally posted by Dwill104:
. Plus, the FE bolt also has a revised angle on the bolt lugs that’s supposed to be better.
Obligatory photo.
I have a legacy ejector bolt with the updated 50 degree bolt angles. You can tell which bolt you have. The newer ones are marked with an engraved numerical 50. It does seem to shoot a little softer. But I never had any trouble with the original bolt.
The FE conversion was a little too salty for my taste, but I understand why some have opted to go that route.
Despite several thousand rounds on both bolts, I haven't had an ejector spring fail yet. Just lucky perhaps.
I love having the same manual of arms of my 5.56 ARs. It just feels natural. So smooth with 147gr subsonic ammo.
I do think the CMMG radially delay system is the sleeper PCC that doesn't get as much attention as it should.
Really nice being able to have one SBR lower that can do 9mm with endomags and any of the other regular small frame AR calibers.
I got the revised 50 degree bolt from CMMG a year or two ago. And the carrier weight kit a year or two before that. It still eats ejector springs. On the upside, I now have a spare bolt that lives in the Magpul MIAD grip on the gun that's pretty easy to swap out in the field if they start dribbling out, and then I go home and swap the ejector spring again. I keep spares on hand so it's no big deal, just a minor annoyance. I still love the gun and am actually considering ordering an 8" bolt/barrel combo to put together an SBR now that the tax stamp is free.
Really has me wondering if there is a tolerance stacking issue somewhere that leads some people to have ejector spring issues whereas others don't.
I'm going to pay close attention to mine over the next 500 rounds and see if it eventually starts having issues.
February 09, 2026, 06:50 AM
92fstechThe only thing that I can figure is that since the bolt doesn't really lock closed like a traditional gas-operated AR, the ejector may not be pressed completely flush to the breech face when the gun fires, and as such it sustains more of a pounding under recoil than a gas gun. Or it could be something with the rim thickness of 9mm ammo as it relates to the extractor claw, which may not be pinning the case flush to the the breech every time. I shoot pretty much 100% reloads through mine, and I don't really pay any attention to the head stamp for pistol stuff, so I'm sure there's a pretty significant variance between rim thicknesses.
Ultimately I haven't really put as much effort into troubleshooting it as I probably should because the interval between problems is long enough and predictable enough to not be much of an inconvenience, and the repair is cheap and easy.
-----------------------------------------------------------
Any comments made by this poster are my own and do not reflect the views or opinions of my employer.
February 09, 2026, 06:30 PM
IndianaBoyquote:
Originally posted by 92fstech:
The only thing that I can figure is that since the bolt doesn't really lock closed like a traditional gas-operated AR, the ejector may not be pressed completely flush to the breech face when the gun fires, and as such it sustains more of a pounding under recoil than a gas gun. Or it could be something with the rim thickness of 9mm ammo as it relates to the extractor claw, which may not be pinning the case flush to the the breech every time. I shoot pretty much 100% reloads through mine, and I don't really pay any attention to the head stamp for pistol stuff, so I'm sure there's a pretty significant variance between rim thicknesses.
Ultimately I haven't really put as much effort into troubleshooting it as I probably should because the interval between problems is long enough and predictable enough to not be much of an inconvenience, and the repair is cheap and easy.
I definitely think that an impulse is being transmitted to the ejector which then compresses the ejector spring beyond what is typical on a standard AR bolt. I wonder if a titanium ejector would lessen the amount of energy being imparted to the spring.
I agree, it's not a deal breaker for the platform.
February 09, 2026, 06:31 PM
IndianaBoyI wonder if a stop inserted inside the spring behind the ejector to keep it from over compressing and over working the spring might help?
February 10, 2026, 02:48 AM
Mars_AttacksA flat spring like a flat buffer spring stops spring overcompression.
____________________________
Eeewwww, don't touch it!
Here, poke at it with this stick.
February 13, 2026, 04:15 PM
Mars_AttacksThe conversion kit arrived today and I have ordered a 5" barrel kit to go on the other receiver.
I noticed the bolt head is no longer spring loaded and there is a slot in the bolt for the fixed side ejector.
____________________________
Eeewwww, don't touch it!
Here, poke at it with this stick.
February 13, 2026, 06:14 PM
92fstechquote:
Originally posted by Mars_Attacks:
A flat spring like a flat buffer spring stops spring overcompression.
I wonder if somebody makes one small enough yet strong enough for the ejector channel. If they do I'd try it.
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Any comments made by this poster are my own and do not reflect the views or opinions of my employer.
February 19, 2026, 05:25 PM
Mars_AttacksMy upgrade kit arrived along with the 5" barrel kit last night.
I REALLY like the new design of the fixed ejector.
____________________________
Eeewwww, don't touch it!
Here, poke at it with this stick.