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MI, USA
Posted: 7/21/2026 7:15:24 PM EDT
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Build: Angstadt PCC Upper Angstadt 0940 Lower 8.5" JRC Long Buffer Tube KynSHOT RB5015HD Hydraulic Buffer CMMG 5" 9mm Barrel & RDB Bolt C3Junkie custom flat-wire spring A few questions/concerns: 1.) The Angstadt 0940 lower says it isn't compatible with the CMMG radial delayed system. If I remove the ejector by pulling the pin, is that all that's required, or is there more to it? 2.) CMMG says the RDB system requires an extended ejection port. I've seen several people say they've run standard 9mm uppers without any issues. Is that actually the case? or is it a guessing game? 3.) I bought the RB5015HD instead of the RB5007. If the gun ends up running reliably with the 5015HD, is there any reason to swap to the 5007, or should I just leave it alone? 4.) With the 8.5" JRC buffer tube, I shouldn't have to worry about excessive bolt travel, correct? I've been reading through C3Junkie's and BB9's posts, and it sounds like spacer weights aren't needed with an RDB setup. Am I understanding that correctly? As you can probably tell, I originally planned on doing a DBB build. Then someone offered me the CMMG barrel and bolt combo for $200, and it seemed too good of a deal to pass up. Now I'm second-guessing whether all of these parts are going to play nicely together, whether it'll run reliably, or if I've overlooked something that could create a serious problem. I'm brand new to the AR9 platform and definitely should've done more research before I started buying parts. I'd really appreciate any input from those of you who have experience with the CMMG RDB system or have run a similar setup. Thanks |
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Sounds like you got the crappy old spring loaded ejector bolt: https://c3junkie.com/?page_id=221 CMMG basically phased those out since they came out with the FE (Fixed Ejector) version in February 2025. Read my site regarding the 'ARS' and the 'ARS-EX' : https://c3junkie.com/?page_id=2757 You would only use the JRC with an A5 length buffer and you would need a spacer. If you don't want to tinker, just stay with the regular ARS and use an A5 length tube and A5 length buffer. If you look at my high speed videos, you will see that I have some setups tuned so that the carrier doesn't bottom out. So if you have a configuration that doesn't bottom out the hydraulic doesn't do much. It is only the initial recoil impulse where the buffer body will move forward and the 5015HD is heavy dampening with a heavier return spring as well so it barely moves at all while the 5007 is lighter dampening so it moves forward more. That is why I prefer the 5007 and to add to that I collaborated with Kynshot. I told them even the 5007 doesn't more forward enough for me either so they came out with the 5007A5 which is going to be released in August. It is targeted for use with tuned 556 and CMMG delayed blowback setups. Also forgot to mention that the hydraulic has works when going into battery to minimize bolt bounce. |
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Yeah, the person who sold you the kit likely bought this and unloaded the in-bolt ejector model on you. They work for about 1000rds and then you have to start replacing ejector parts. Fixed ejector solved the problem |
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