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Posted: 12/31/2025 6:19:05 PM EDT
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I have 3 MP5s with SuperSafeties but now want to build a AR pattern 9mm. I've decided to setup with a RDB system from CMMG. Plan right now; -CMMG fixed ejector upper -8in barrel from Amphibian -3 lug to Omega 9K -Colt mag adapter in an SBR lower -Would like to use a Maxim CQB stock but uncertain if the buffer can get heavy enough. I'll settle for a standard buffer tube. -Partisan Disrupter FRT Thoughts? Missing anything? My rational; RDB has a almost no chance of out of battery detonation unlike a straight blowback 9mm AR. Should be "softer"; closer to the MP5 recoil. Traditional bolt carrier means no trip timing issues. Colt mags because they look good. |
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Quoted: Give the Kynshot hydraulic buffer a try. It gives you a ton of dead blow, or damping effect, without putting 200 bucks worth of Tungsten back there. Being an RDB I shouldn't need as heavier of a buffer I thought.... Off to go reread parts of Amphibian's info. |
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Quoted: Being an RDB I shouldn't need as heavier of a buffer I thought.... Off to go reread parts of Amphibian's info. Quoted: Quoted: Give the Kynshot hydraulic buffer a try. It gives you a ton of dead blow, or damping effect, without putting 200 bucks worth of Tungsten back there. Being an RDB I shouldn't need as heavier of a buffer I thought.... Off to go reread parts of Amphibian's info. Oh, note that I don't have an RDB. Just a general 9mm thing. I don't have personal experience with that platform. |
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Quoted: Oh, note that I don't have an RDB. Just a general 9mm thing. I don't have personal experience with that platform. Quoted: Quoted: Quoted: Give the Kynshot hydraulic buffer a try. It gives you a ton of dead blow, or damping effect, without putting 200 bucks worth of Tungsten back there. Being an RDB I shouldn't need as heavier of a buffer I thought.... Off to go reread parts of Amphibian's info. Oh, note that I don't have an RDB. Just a general 9mm thing. I don't have personal experience with that platform. As I understand radial Delayed Blowback, it's a rotating AR style bolt and regular carrier but the lugs are radiused so that the case pushing back unlocks the bolt. Seems perfect for an FRT. |
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Quoted: As I understand radial Delayed Blowback, it's a rotating AR style bolt and regular carrier but the lugs are radiused so that the case pushing back unlocks the bolt. Seems perfect for an FRT. Quoted: Quoted: Quoted: Quoted: Give the Kynshot hydraulic buffer a try. It gives you a ton of dead blow, or damping effect, without putting 200 bucks worth of Tungsten back there. Being an RDB I shouldn't need as heavier of a buffer I thought.... Off to go reread parts of Amphibian's info. Oh, note that I don't have an RDB. Just a general 9mm thing. I don't have personal experience with that platform. As I understand radial Delayed Blowback, it's a rotating AR style bolt and regular carrier but the lugs are radiused so that the case pushing back unlocks the bolt. Seems perfect for an FRT. Yeah, that sounds great. Especially for a silencer, keep the port pop low. |
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Quoted: Give the Kynshot hydraulic buffer a try. It gives you a ton of dead blow, or damping effect, without putting 200 bucks worth of Tungsten back there. Quoted: Give the Kynshot hydraulic buffer a try. It gives you a ton of dead blow, or damping effect, without putting 200 bucks worth of Tungsten back there. I know OP is looking at a radial-delayed system rather than a straight blow-back, but https://blowback9.wordpress.com/ has a ton of info on hydraulic buffers. In my own (very limited) experience, the CMMG RDB with an H3 is softer shooting than a straight blow-back with a 10 oz PCC buffer (i.e., 24+ oz. total reciprocating mass). I've never tried the Banshee with a hydraulic buffer, though. From 9mm AR Gentle Recoil Buffer System: Forced Reset, Super Safety, Full Auto, Binary, etc.: I have seen one report of out-of-battery detonations when using the Gentle Recoil System with a Super Safety. Try at your own risk. Assuming the RDB design mitigates that risk, it sounds like it could be pretty sweet. UPDATE: Someone's already tried this: From 9mm AR Gentle Recoil Buffer System: CMMG Banshee (ONLY) Credit for this combination goes entirely to “Amphibian-C3Junkie” who has recently coined it the “Amphibian Recoil System” or ARS to distinguish it from my blowback GRS. He is a Banshee expert and landed on this combination before the Gentle Recoil System was a thing. We both reference each other’s research, and collaborated with each other on ideas. I’m including it here since it has come up a few times. More can be found on his C3 Junkie CMMG ARS page. This system is NOT appropriate for blowback firearms. It is only for the Banshee RDB. |
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I have two: 1) CMMG 8” barrel with original plunger ejector bolt in a Gibbz side charging upper. FRT is an Atrius Super Selector behind a BattleHawk (Rise?) cassette trigger. 2) CMMG 11.5” barrel (cut down from a 16” plunger ejector barrel by ADCO) with a fixed ejector conversion bolt and upper. FRT is an ARCFire with the supplied GI trigger. Both are on SBR Glock magazine lowers with carbine buffer tubes. The 8” needed an H2 buffer to run, but the 11.5” runs great with a regular carbine buffer… my guess is the bolt lug angles are different between the plunger ejector and fixed ejector bolts. This is an incredibly flat shooting system, better than an MP5 IMHO |
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Quoted: I have two: 1) CMMG 8” barrel with original plunger ejector bolt in a Gibbz side charging upper. FRT is an Atrius Super Selector behind a BattleHawk (Rise?) cassette trigger. 2) CMMG 11.5” barrel (cut down from a 16” plunger ejector barrel by ADCO) with a fixed ejector conversion bolt and upper. FRT is an ARCFire with the supplied GI trigger. Both are on SBR Glock magazine lowers with carbine buffer tubes. The 8” needed an H2 buffer to run, but the 11.5” runs great with a regular carbine buffer… my guess is the bolt lug angles are different between the plunger ejector and fixed ejector bolts. This is an incredibly flat shooting system, better than an MP5 IMHO Thanks. |
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Quoted: @amphibian Any input as the resident RDB guru? I've been reading up on your c3junkie blog. I don't keep up with the Maxim CQB stock but taking a quick look, it appears there are several options. I don't think any of them will be as smooth as what I recommend here: https://c3junkie.com/?page_id=2757 I have a bunch of people run FRTs and they have been drop in. I'm in Florida where they are illegal so have no first hand experience with them. |
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Quoted: I have two: 1) CMMG 8” barrel with original plunger ejector bolt in a Gibbz side charging upper. FRT is an Atrius Super Selector behind a BattleHawk (Rise?) cassette trigger. 2) CMMG 11.5” barrel (cut down from a 16” plunger ejector barrel by ADCO) with a fixed ejector conversion bolt and upper. FRT is an ARCFire with the supplied GI trigger. Both are on SBR Glock magazine lowers with carbine buffer tubes. The 8” needed an H2 buffer to run, but the 11.5” runs great with a regular carbine buffer… my guess is the bolt lug angles are different between the plunger ejector and fixed ejector bolts. This is an incredibly flat shooting system, better than an MP5 IMHO I've built two 9mm RDB uppers (w/out the newer fixed ejectors) and tested them on my AR15 SS lower with amphibian's A5 recipe (Flatwire, Kynshot rb5007 & A5 RE). I agree ?? that it's flatter than the MP5. I tested them back to back with a full size MP5 w/Leber SS lower I built. Cyclic rate on the RDB was much slower and comparable to the SCAR16 ker-klunk action; whereas, the MP5 cyclic rate was much faster but still very controllable. I actually struggled to determine which one I liked more that day of testing because both were equally fun to shoot. |
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Quoted: Make sure you read my page regarding Colt mags and the CMMG RDB system: https://c3junkie.com/?page_id=374 I don't keep up with the Maxim CQB stock but taking a quick look, it appears there are several options. I don't think any of them will be as smooth as what I recommend here: https://c3junkie.com/?page_id=2757 I have a bunch of people run FRTs and they have been drop in. I'm in Florida where they are illegal so have no first hand experience with them. Which FRTs are they running? Especially 9mm, CMMG FE, suppressor. |
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I have a 5" 9mm glock mag Banshee FE with a KVP ASR brake and FRT15-L3 You will need a standard, H2 and an H3 buffer depending on the ROF you want, type of ammo you want, and suppressed vs loud. Use the standard carbine spring that comes with the Banshee. I find that using 147gr subs with a SiCo Hybrid .46 and an H2 buffer is the sweet spot for suppressed. H3 works very well but is a slow, chugging ROF. H1 you will get hammer follow and a dead trigger with live rounds in the chamber. There really wasn't any ammo/suppressor combination that made it worth having an H1. It will run 115gr ammo with a standard carbine buffer unsuppressed and a much faster ROF. Adding a heavier buffer without a suppressor will prevent locking open on empty mags. You need the suppressor back pressure to help with this. Really it's a balancing act, but once you decide on an ammo, you can find the best buffers to use for loud vs suppressed. Suppressed, it's very dirty, but only half as dirty as a straight blowback. All that said, your 8" barrel might throw all that out the window as you'll have more back pressure.  One good thing about the RDB is you don't need longer, heavier 9mm blowback buffers. And it is easier on the FCG because the bolt is slowed by the radial delay. Straight blowback 9mms have had a reputation for being hard on cartridge triggers. |
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I run the Broad River Tactical 5" ported barrel/SD chamber + suppressor with the CMMG external ejector, radial delayed, with a Sprinco yellow buffer spring and 3oz. buffer. Everything that exits the muzzle is subsonic. I also run hot hand loads, 147 grain supersonic with Nas3 bi-metal cases so it's a different animal but super quiet and ear safe. It works with a Hoffman-style super safety but not as well with Arc-fire in fun mode and not at all with an Atrius. I have yet to try my Rare Breed or Arizona in it just yet. But the radial delayed BCG's, as others have said, don't require a heavy buffer. I'd maybe get an Odin Works adjustable buffer so you can mess around with buffer weights easily. They're not that expensive and it's easy to change out weights. Just don't use the included felt pads in between the weights so it'll have a dead blow effect. Less overall mass does help reduce recoil and it's extremely easy to stay right on target. |
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Quoted: I'd maybe get an Odin Works adjustable buffer so you can mess around with buffer weights easily. They're not that expensive and it's easy to change out weights. Did you get anything to run with an Odin "Heavy" buffer? I thought the same. The buffer doesn't brrrrt with 1, 2 or 3 Tungsten IME. I am going to try the Odin "Light" next time out. But I'm taking H3s and H2s with sliding weights as well. Steel bounces; aluminum deforms. If it was bounce caused by the steel body, the "Light" should work as you've described. But it'll still be rely'g on the cap screw to stay outa the way. |
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I wasn't able to get my 45 RDB to work with the Maxim stock due to the spring loaded bolt. ETA: To be clear, it assembles fine, there are no clearance issues, but the short Maxim spring was not always enough to overcome the spring loaded bolt, so it would not always go in to battery completely. At the time there were no other spring options from Maxim, maybe there are now. I just put a carbine stock/h2/carbine spring and it worked flawlessly. |
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@blowback9 @blowback_9 @blowback_9 @blowbacknine I had finally memorized droppoint and then you go & change it. OP, if it's a range toy or HD gun, why not a fixed stock? You'll have a better cheekweld, if you go to the Vism thumbhole, short, but you can tune your safety detent spring to your liking. |
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+1 for Amphibian's RDB recipe (A 5 tube, flatwire spring and the Kynshot buffer). I built mine based on his input, except I bought a 5" barrel from him with a single feed ramp. I run it on my SBR lower with an Endomag. The thing is fricken sweet. I went with the Neverwear Warthog spring as I swap the 9mm upper back and forth with a 7.5" upper in 300 Blk with an AGB. Works like a charm. I'm still running a binary trigger in it for now, but it just works...smooooth.  Here's my son ripping off 11 rounds with the 9mm. No muzzle flip to speak of, and not much recoil. (sorry it's a "youtube short" video, it seems ARFCOM's embed fucntion doesn't work with youtube short videos, so it's just a link). https://youtube.com/shorts//HHPArWDq8Uw |
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Quoted: ... Here's my son ripping off 11 rounds with the 9mm. No muzzle flip to speak of, and not much recoil. (sorry it's a "youtube short" video, it seems ARFCOM's embed fucntion doesn't work with youtube short videos, so it's just a link). https://youtube.com/shorts//HHPArWDq8Uw Fixed that for ya:  CMMG RDB 9 |
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