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Posted: 7/8/2025 9:42:46 PM EDT
[Last Edit: kcobean][Edited]
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I have a CMMG Colt pattern AR9 with a 4.5” barrel and a Surefire can. I recently swapped my DTT FRT into this gun from my AR15 and while it does hand-cycle properly, it will not cycle properly during firing. I filmed the bolt closing from the bolt catch and it does not appear that I’m getting bolt bounce, but I’m getting light strikes in the second round. Buffer is a Spikes ST9X. Any thoughts? |
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Originally Posted By kcobean: I have a CMMG Colt pattern AR9 with a 4.5” barrel and a Surefire can. I recently swapped my DTT FRT into this gun from my AR15 and while it does hand-cycle properly, it will not cycle properly during firing. I filmed the bolt closing from the bolt catch and it does not appear that I’m getting bolt bounce, but I’m getting light strikes in the second round. Buffer is a Spikes ST9X. Any thoughts? Slow it down or speed it up and see what happens, if it's standard blowback I'm guessing you don't need to speed it up. |
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| CMMG sells weights that slide into the bolt carrier and are secured by a large roll pin. My CMMG Banshee runs very well now with a binary trigger but only after I changed the buffer to a heavier buffer with sliding weights inside, and added the CMMG weight to the bolt carrier. I have a RB FRT on the way to me now and I plan on replacing the binary trigger with the FRT. I will let you know if it continues to run well with the FRT with the same buffer and weight set up. |
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Originally Posted By kcobean: What's the best way to do that? The ST9x is the heaviest buffer I know of. https://kakindustry.com/9mm-carbine-buffer-extended-heavy |
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If that's a solid buffer you're running, you are getting bolt bounce. You just aren't seeing it. Your timing is off. My guess is too much bolt velocity and bolt bounce. When I was running a blowback setup, I used a Vltor A5SH4 buffer. Its an all stainless body with sliding weights. With the suppressor, its likely a good idea to add the weights to the carrier and run a springco blue or maybe even a red spring to slow down that bolt velocity. I'd start with the buffer and see what that gets you. As far as that delayed buffer goes, the one with the bearings in it, they work, but the effort needed to charge the weapon almost gets into the stupid range. I think I saw a video where a guy did a test and said it required in excess of 30 lbs of force to open it. I didn't think it felt that heavy, but doing drills on induced malfunctions was not fun or fast. The cost of getting an AR9 up and running starts to add up and you may never get to the level of reliability that you want. I chose to upgrade to a CMMG radial delayed system. Mine is the older version with the ejector in the bolt. It cost me less than $300 to do. The only downside to it is that the ejector springs wear out fast. How fast depends on which spring you use. I'm running the CMMG Mk10 springs and just replace it when I have to reorder ammo. That equates to about every 2000 rounds. |
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Originally Posted By Cpatterson: Maxim Defense's Roller Delayed Buffer (RDB)  I went this way with an 8" ar9 with a SS. It would only run 10ish rounds. It's either hammer follow or bolt bounce. Dies hammer down with a round in the chamber. Changed to the 10oz buffer and it runs without issue. |
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I run Stoner 10oz 9mm buffer with Strike flat carbine spring in all my 9mm builds and have had great results,no issues RTB 11” upper there bolt and 8.3” BK4 there bolt Blowback 9 has all the info you want or need stick with his formula and ya cant go wrong,,in my opinion and i gota say for the money RTB uppers are soild and well done,well worth your time to watch there sales |
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