- Thread starter supercorndogs
- Start date
I've got a 9mm CMMG guard (radial delayed blowback, or RDB) barrel & bcg in a standard AR upper & lower (carbine buffer & spring); it's so much better than straight blowback it's not funny. Recoil is really mild, unlike blowback guns where it's worse than 5.56 because of the heavy mass; this one feels like a braked 5.56 without the muzzle blast. It's also a very light rifle, which you can't do with a blowback gun. It's also the quietest suppressed AR15 I've ever used or heard (with subs of course); much more so than any blowback 9mm and also quieter than any of my 300 Blk sub setup. I'll probably convert my blowback 45 to this system as well.
Another plus of the RDB system is the ejector is in the bolt like a 5.56, so I don't need an ejector mounted in the lower. I'm using MEAN Endomags (inserts inside 30rd Pmags) without the blowback ejector and really like them. And I'm using them and this upper with any of my 5.56 lowers, since it doesn't need a heavy recoil system; that's handy when you have some SBR lowers already to use with a short barrel upper.
I've read some older reports of the CMMG system not being very accurate, but mine is a tackdriver with both jacketed and cast bullet loads. Maybe they've changed something there, not sure, but would definitely buy another like mine.
I've also read older reports from the same people about the ejector springs going soft in these CMMG bolts. Not sure if mine will do that (only have a few hundred rounds through it), but I'll probably strip it apart first to install a pin behind the ejector to prevent over-compressing the spring. That should solve any issues, if CMMG hasn't done it already.
Edit - I should add that I've had 9mm blowback systems before, so the above is a valid comparison. Obviously the AR platform is the intended comparison, but it's also worth throwing out others like the Scorpion, Beretta CX4 Storm, Uzi, Mac, and even the old MechTech system. The MechTech felt the mildest of those blowback systems to me (including the ARs) but the RDB system blows them all out of the water. It feels more like shooting an MP5.
Another plus of the RDB system is the ejector is in the bolt like a 5.56, so I don't need an ejector mounted in the lower. I'm using MEAN Endomags (inserts inside 30rd Pmags) without the blowback ejector and really like them. And I'm using them and this upper with any of my 5.56 lowers, since it doesn't need a heavy recoil system; that's handy when you have some SBR lowers already to use with a short barrel upper.
I've read some older reports of the CMMG system not being very accurate, but mine is a tackdriver with both jacketed and cast bullet loads. Maybe they've changed something there, not sure, but would definitely buy another like mine.
I've also read older reports from the same people about the ejector springs going soft in these CMMG bolts. Not sure if mine will do that (only have a few hundred rounds through it), but I'll probably strip it apart first to install a pin behind the ejector to prevent over-compressing the spring. That should solve any issues, if CMMG hasn't done it already.
Edit - I should add that I've had 9mm blowback systems before, so the above is a valid comparison. Obviously the AR platform is the intended comparison, but it's also worth throwing out others like the Scorpion, Beretta CX4 Storm, Uzi, Mac, and even the old MechTech system. The MechTech felt the mildest of those blowback systems to me (including the ARs) but the RDB system blows them all out of the water. It feels more like shooting an MP5.
Last edited:
So about the only draw back is I woukd have to snip the ejector off my dedicated lower or get another.
yep, basically the RDB system is a way of giving the AR platform the benefits of an MP5's delayed blowback without having to fuck with the receivers; just the bolt, carrier, and barrel extension.
if you go with a new dedicated lower, consider looking for one that does MP5 or colt SMG mags, as both are double stack/double feed and will be generally more reliable than glock mags for the purpose
New frontier armory has one that rolls MP5 mags: https://www.newfrontierarmory.com/g-5-billet-lower-gen-2/
Colt SMG you'd just roll a normal lower with adapter block (ideal if you want to swap it back to a centerfire caliber upper)
Haven't shot a 9mm AR RDB like the JP or CMMG but the difference between a HK SP5 and CZ Scorpion was noticeable. SP5 (minus the meh trigger) was more enjoyable to shoot.
I haven't shot a B&T with their hydraulic buffer so I wonder if that brings the blowback closer to the RDB as far as felt recoil.
I haven't shot a B&T with their hydraulic buffer so I wonder if that brings the blowback closer to the RDB as far as felt recoil.
The JP systems sounds like it's the premium way to go if you have the cash. But IMO this CMMG setup is like an MP5 but with a good AR trigger.
On the B&T - hydraulic buffer or not, the heavy mass required of a straight blowback system is always a significant factor. Delayed blowback (whether that's roller, radial, or even gas delay like an HK P7, or other systems) reduces or eliminates the need for that extra mass. I'm using this CMMG system with a standard weight carrier and standard carbine spring and buffer. They have a hole in the carrier to add extra weight, but it doesn't seem to be needed for my 9mm subs.
Most 9mm lowers have the ejector held in place with a set screw or something similar and can be removed.
But if you noticed my comment about the Endomags, you can use any 5.56 lower with those. I like them better than Colt or MP5 mags anyway; they have the same feed advantages (still double feed like the regular Pmag, they use the Pmag feed lips) but also have the advantage of the same feel and mechanics as a 5.56 rifle. And they're a lot cheaper than other SMG mags anyway, and don't need a mag adapter block. The standard bolt hold open works with these too, which is often an issue with other 9mm AR mag types.
To be clear - I don't sell these or have any financial stake in them; just pointing out that they seem to have all of the answers we look for in 9mm AR systems, IF you're using a CMMG or other system that has an ejector in the bolt. For blowback systems, they do make a version that has a mag-mounted ejector, but I'm not real excited about that idea - it's plastic, and you can't eject a round after dropping the mag in that blowback version.
I have a Glock mag lower too and a handful of 33rd happy sticks, but not sure there's any good reason to use it after getting set up with these Endomags. Maybe with one of the Glock-compatible drum mags there could be an advantage, but I haven't tried them and not sure if they have durability/reliability issues in ARs; it seems to depend who you ask.
BTW @WindstormSCR - the barrel extension in this CMMG system is only different from a 5.56 in the feed ramp - it's got single wide feed ramp that accomodates both single feed and double feed. Apparently CMMG's original design included angled lug faces, but they're building them with standard flat faces. Pretty easy to make the barrel itself using a 5.56 extension if a guy has the ability; it'd be essentially the same as a 9mm DI barrel but without a gas port.
Adding a couple pics of a MEAN Endomag. You can buy them complete, or just the guts and load them into your own stripped Pmags. IIRC they were around $30 each for the guts, and Pmags can be had for $10-$12; a lot cheaper than Colt mags by a long shot. Also, they handle a bit longer OAL than normal 9mm mags; not a huge difference but maybe another .100" or so, enough to open up some more options with heavy bullet subsonic loads.
Last edited:
Most might have a removeable ejector but mine doesn't.
Do the endo mags work with 40s&w?
Do the endo mags work with 40s&w?
@Yondering thanks for the info about the extensions, you’d think that would cause significant malfunctions.
I’ll drop a bonus in the thread: PCC specific load meant to enulate 9mm NATO high pressure subgun ammo without actually being hotter than +p:
NAS3 Shellshock bimetal cases, 90gr sig V-crown over 11gr of accurate No. 7, loaded to about 1.080 coal (v-crown should be sitting on top of the powder, its a 100% volume load)
out of a KP9 barrel it goes about 2020fps, an MP5 barrel a little less, a longer AR9 barrel it tops out about 2350 fps. you will feel recoil
I’ll drop a bonus in the thread: PCC specific load meant to enulate 9mm NATO high pressure subgun ammo without actually being hotter than +p:
NAS3 Shellshock bimetal cases, 90gr sig V-crown over 11gr of accurate No. 7, loaded to about 1.080 coal (v-crown should be sitting on top of the powder, its a 100% volume load)
out of a KP9 barrel it goes about 2020fps, an MP5 barrel a little less, a longer AR9 barrel it tops out about 2350 fps. you will feel recoil
The extensions as they are seem to work fine, no issues. I've noticed some scratching on extracted cases as they rub against the sharp edge of one lug, but it's common to see the same thing in 5.56 too; easily fixed by polishing down that sharp edge. No other malfunctions to report.
It seems that it'd make sense to have angled faces on the extension lugs, but machining those is significantly more difficult and expensive than ordinary flat lug faces, so my assumption is CMMG discovered it worked fine this way and went with it. There is a small chamfer or broken edge on the contact edge of course, same as most quality 5.56 extensions; that surface contacts the angles on the bolt lugs.
A note on the bolt lug angles - they use different angles depending on power/pressure of the cartridge (it's not just peak pressure & bolt thrust, but duration as well that matters), so the 40 S&W bolts have a shallower angle (closer to standard flat bolt lug) than 9mm, and the 10mm and 45 have even shallower angles than that. I saw mention in some older discussion of these about someone using a 40S&W bolt for their hotter 9mm loads; that probably makes a lot of sense if that's what you want to shoot. It'd be a good idea for something like the load pictured above; even though it's not high pressure (judging by the primers) the duration of pressure has to be relatively long, causing more recoil in a standard blowback system and probably more aggressive "unlocking"* in this RDB system too. Personally I'm using this for suppressed subsonic loads so this setup is fine, but may experiment with hotter loads at some point and maybe a 40 S&W bolt.
Good question - hadn't really thought about it before. They're marketed as 9mm mags so it'd be a "try at your own risk" situation of course, but I just loaded a handful of 40 S&W and 357 Sig rounds into one of them and it seemed to hold and feed them fine by hand. Being double feed there's a little more forgiveness there. I don't have a 40 S&W AR carbine to try them in, but cycled some by hand through my 45 AR and they seemed to feed OK, for whatever that's worth.
So, maybe? Certainly not a hard no anyway; it'd probably be worth trying one mag at least.
I had a blowback AR9 and replaced it with Foxtrot Mikes FM9 lower with a RDB upper I built myself and I'd say it's got 1/3 the recoil of my blowback setup and I would never go back.
I also have an MKE AP5 and it's got less recoil than the RDB setup by I'd say 10-20%. The roller delay setup recoil does feel smoother and slower cycling than the RDB setup.
I also have an MKE AP5 and it's got less recoil than the RDB setup by I'd say 10-20%. The roller delay setup recoil does feel smoother and slower cycling than the RDB setup.
Sounds about right.
CMMG does sell "tuning kits" for the RDB system (different weights that fit inside the bolt carrier) which would probably make it feel a lot more like that slower and smoother cycling of the MP5 system that you mentioned. Depends on the loads you're shooting though; if they're fairly mild to be quiet subsonics, IMO the lighter carrier is better, but added mass would help for hotter loads.
Tuning with different bolt angles would be the better way to go though IMO, same effect as changing the MP5 locking piece for a different angle to adjust the delay; if you can tune for the right amount of delay while keeping the reciprocating mass low, it'll give the softest shooting feel. CMMG does sell two different 40S&W bolts for 8" and 16" barrels that should be good options for tuning the 9mm system for heavier loads; Brownells has them for about $155 or direct from CMMG for $170.
