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Posted: 7/21/2026 2:50:32 PM EDT
[Last Edit: bending_rodriguez][Edited]
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So this question is really directed towards amphibian, but anyone who might know the answer is welcome to help! I have followed your C3junkie 9mm RDB write ups since the beginning. I’ve been using your exact setup for years now, but lately I’m getting tired of getting screwed up in Subgun matches because of a failed ejector spring. My question that I can’t seem to figure out on your site is this…. Does the newer fixed ejector retrofit kit work as is, as far as replacing the upper receiver and bcg? I can’t figure out if your phase I and phaseII are prior to the release of the retrofit kit. Will the new cmmg upper and bcg drop in and work the same way as the older rdb setup worked, aside from the change in ejectors? Will the tubbs flatwire spring and kynshot hydraulic buffer still work with this retrofit setup, regarding smooth cycling and slower rof? Does the new bcg trip the sear? Is it worth the swap over to the fixed ejector system as opposed to just continuing with the spring ejector and just replacing the spring every 800 rounds or so? |
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^im not sure about the lower, I’m using this setup on a RR. I guess my OP might be worded a little confusing. Will the new fixed extractor upper receiver and bolt function similarly to the original RDB setup, if I continue to use the a5 length tube, tubbs spring, and kynshot hydraulic buffer in a RR? Are there any downsides moving to the fixed ejector setup? |
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Originally Posted By bending_rodriguez: ^im not sure about the lower, I’m using this setup on a RR. I guess my OP might be worded a little confusing. Will the new fixed extractor upper receiver and bolt function similarly to the original RDB setup, if I continue to use the a5 length tube, tubbs spring, and kynshot hydraulic buffer in a RR? Are there any downsides moving to the fixed ejector setup? I have built two CMMG fixed ejector uppers in the past year using their fixed ejector upper "kit". Both of those fixed ejector kits were paired with a A5 length tube, the full size Kynshot 9mm buffer, and a flatwire spring (Botach vs. Tubb spring if memory serves correctly). The fixed ejector kits will run out of the box on an M16 unlike the previous "dissent" bufferless RDB fixed ejector uppers that amphibian customized to work with a buffer tube and auto sear. We are both running Colt mag setupsthat did require the CMMG specific mag block feed ramp and magazine catch that allow the Colt Mags to sit every so slightly lower in the lower receiver. So if you are running a registered receiver M16 you will need to swap out the mag release or use PMags with the Endo Mag insert that will run without the CMMG specific mag catch. (I think that was a limitation on the original RDB uppers as well) Both of the uppers I built (one for myself and one for a friend) the Fixed Ejector Kit carriers were both cut back to SP1 spec for use with Lightning Links. That way we could have dedicated SBR lowers, running Colt mags, the CMMG specific catch and feed ramp components and just drop a link into the SBR host for full auto. I never owned a non-fixed ejector kit but my buddy had one of the original spring loaded ejector units that was flaming dog shit and ate extractors every couple hundred rounds. The new FE units so far are really great. The FE kits are reported to break firing pins but neither of us have broken an FE kit FP yet but we do have all the parts on hand to move to the Amphibian recommended TI FP with the collar to get rid of the FP spring if/when our FPs break. I like mine so much I plan to buy a 45ACP FE kit and build a dedicated SBR host lower using the Hydra Lower with a Grease Gun mag adapter, and make a custom co-axial low-back pressure 45ACP subgun suppressor for it. I am really hoping they come out with a 10MM FE kit at some point in the future. |
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Originally Posted By jbntex: I have built two CMMG fixed ejector uppers in the past year using their fixed ejector upper "kit". Both of those fixed ejector kits were paired with a A5 length tube, the full size Kynshot 9mm buffer, and a flatwire spring (Botach vs. Tubb spring if memory serves correctly). The fixed ejector kits will run out of the box on an M16 unlike the previous "dissent" bufferless RDB fixed ejector uppers that amphibian customized to work with a buffer tube and auto sear. We are both running Colt mag setupsthat did require the CMMG specific mag block feed ramp and magazine catch that allow the Colt Mags to sit every so slightly lower in the lower receiver. So if you are running a registered receiver M16 you will need to swap out the mag release or use PMags with the Endo Mag insert that will run without the CMMG specific mag catch. (I think that was a limitation on the original RDB uppers as well) Both of the uppers I built (one for myself and one for a friend) the Fixed Ejector Kit carriers were both cut back to SP1 spec for use with Lightning Links. That way we could have dedicated SBR lowers, running Colt mags, the CMMG specific catch and feed ramp components and just drop a link into the SBR host for full auto. I never owned a non-fixed ejector kit but my buddy had one of the original spring loaded ejector units that was flaming dog shit and ate extractors every couple hundred rounds. The new FE units so far are really great. The FE kits are reported to break firing pins but neither of us have broken an FE kit FP yet but we do have all the parts on hand to move to the Amphibian recommended TI FP with the collar to get rid of the FP spring if/when our FPs break. I like mine so much I plan to buy a 45ACP FE kit and build a dedicated SBR host lower using the Hydra Lower with a Grease Gun mag adapter, and make a custom co-axial low-back pressure 45ACP subgun suppressor for it. I am really hoping they come out with a 10MM FE kit at some point in the future. I'm about to release an adjustable 9mm SD setup. I'm also doing a .45 SD setup as well but not going to make the .45 version adjustable. This link goes over what I did chronologically: https://c3junkie.com/?page_id=3168 |
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Originally Posted By jbntex: I have built two CMMG fixed ejector uppers in the past year using their fixed ejector upper "kit". Both of those fixed ejector kits were paired with a A5 length tube, the full size Kynshot 9mm buffer, and a flatwire spring (Botach vs. Tubb spring if memory serves correctly). The fixed ejector kits will run out of the box on an M16 unlike the previous "dissent" bufferless RDB fixed ejector uppers that amphibian customized to work with a buffer tube and auto sear. We are both running Colt mag setupsthat did require the CMMG specific mag block feed ramp and magazine catch that allow the Colt Mags to sit every so slightly lower in the lower receiver. So if you are running a registered receiver M16 you will need to swap out the mag release or use PMags with the Endo Mag insert that will run without the CMMG specific mag catch. (I think that was a limitation on the original RDB uppers as well) Both of the uppers I built (one for myself and one for a friend) the Fixed Ejector Kit carriers were both cut back to SP1 spec for use with Lightning Links. That way we could have dedicated SBR lowers, running Colt mags, the CMMG specific catch and feed ramp components and just drop a link into the SBR host for full auto. I never owned a non-fixed ejector kit but my buddy had one of the original spring loaded ejector units that was flaming dog shit and ate extractors every couple hundred rounds. The new FE units so far are really great. The FE kits are reported to break firing pins but neither of us have broken an FE kit FP yet but we do have all the parts on hand to move to the Amphibian recommended TI FP with the collar to get rid of the FP spring if/when our FPs break. I like mine so much I plan to buy a 45ACP FE kit and build a dedicated SBR host lower using the Hydra Lower with a Grease Gun mag adapter, and make a custom co-axial low-back pressure 45ACP subgun suppressor for it. I am really hoping they come out with a 10MM FE kit at some point in the future. Awesome, thanks for the info! I got a little confused with amphibians write ups, because he was modifying the original dissent system. It seems like the most recent iteration of the rdb fe works well with no modifications in an m16, aside from the fp issue. If I understood that right, an m16 RR using the pmag endomag inserts, do not need any modification to the carrier? And it should also cycle well with the a5 tube, flatwire spring and kynshot hydraulic buffer? If I have issues with the FP it’s good to know amphibian has the remedy already. ETA thanks for responding amphibian. Also, is cmmg the best place to buy the retrofit kit? That 15-30 day shipping is making me nervous that I won’t have the parts for the my next match. |
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Originally Posted By bending_rodriguez: I got a little confused with amphibians write ups, because he was modifying the original dissent system. It seems like the most recent iteration of the rdb fe works well with no modifications in an m16, aside from the fp issue. Originally Posted By bending_rodriguez: I got a little confused with amphibians write ups, because he was modifying the original dissent system. It seems like the most recent iteration of the rdb fe works well with no modifications in an m16, aside from the fp issue. If I understood that right, an m16 RR using the pmag endomag inserts, do not need any modification to the carrier? And it should also cycle well with the a5 tube, flatwire spring and kynshot hydraulic buffer? This is the only 9mm SMG I know of (open or closed bolt) that can run at less than 600RPM and have a total reciprocating mass of only 12.7oz. If I have issues with the FP it's good to know amphibian has the remedy already. ETA thanks for responding amphibian. Also, is cmmg the best place to buy the retrofit kit? That 15-30 day shipping is making me nervous that I won't have the parts for the my next match. |
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I saw on your site a while back you were using the magnetic buffers vs the hydraulic. I have personally kept with the hydraulic as I was concerned about bolt carrier bounce on full auto with those magnetic buffers. I have even had issues with bolt bounce with the hydraulic buffers in the past as they don't have the bolt carrier bounce dampening effect that traditional H series buffers do. Fortunately, the only version of the Kynshot hydraulic buffer I have not had bolt carrier bounce issues with is the full size 9mm Kynshot version. I suspect this is due to the larger/heavier body on the full size 9mm that give more dampening effect. The worst are the shorty 308 versions which won't run at all in full auto. On full auto 308s running the kynshot shorty buffers I have to make custom "in carrier" anti-bounce inserts with reciprocating tungsten weights to deal with bolt carrier bounce if I want the dampening properties of the hydraulic buffer. Its interesting to see the miculek magnetic buffer had bolt carrier bounce issues and both the miculek and its predecessor both structurally failed. Do you know if Kynshot going to commercially market the softer full size 9mm hydraulic they gave you? |
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Originally Posted By jbntex: Its interesting to see the miculek magnetic buffer had bolt carrier bounce issues and both the miculek and its predecessor both structurally failed. Originally Posted By jbntex: Its interesting to see the miculek magnetic buffer had bolt carrier bounce issues and both the miculek and its predecessor both structurally failed. Do you know if Kynshot going to commercially market the softer full size 9mm hydraulic they gave you? |
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Good to hear on the lighter compression 9mm Kynshot buffers. I always figured since the full size 9mm was built for direct blowback 9mm that it was probably not ideal and a bit stiff for the lower bolt velocity RDB with its built in delay. Having a more RDB specific Kynshot full size 9mm option would be nice option. I will keep an eye on your site in the coming months for them to become available. |
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Originally Posted By Chowser: Would you recommend a Kynshot hydraulic buffer for owners of the original rdb Banshees? |
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