Is the CMMG rotary delayed system worth it?

[GunsNstuff]

Anyone have the CMMG RDB barrel & BCG & also use the Mean Arms pmag conversions? My AR9 has been a 7.5" blowback with adjustable pistol brace and a Glock mag lower, but I've recently replaced the lower with a standard lower and I have the Mean Arms pmag conversions. The problem I'm having is the tiny feeding cone on my barrel. It sometimes loads when I charge the pistol by hand, but when I fire it, the next cartridge gets jammed into the vertical flat surface of the barrel which jams the bullet deep into the case, ruining the cartridge and jamming the weapon.

I thought I'd take a Dremel and add a feed ramp to the lower portion of the barrel so these rounds will be forced to point up and hopefully load into the chamber. My barrel doesn't have an index pin, so I bought some 1/8" stainless rod, drilled an 1/8" hole in the barrel, heated the barrel and froze the rod, then pressed it into the hole in the barrel. Now I have an index pin and tightening the barrel nut won't spin the barrel at all, so I know where to work on adding the feed ramp.

As fun as this work is for me, it's not fun to drive the 35 miles to the property where I shoot to see if a mod I did made the gun work or not. I started thinking that while I think the price of $475 is ridiculous for a BCG and 8" barrel from CMMG, it would make the gun lighter weight and probably make it function properly again since CMMG sells completes AR9s with the Mean Arms mag conversions. It may be a buy once, cry once situation, but since I've never fired one of these radial delayed AR9s, I'm wondering what others think about them. Are they so much better than standard blowback AR9s that the cost is justified, or is it mainly hype? Is the CMMG system pretty reliable? My AR9 with Glock mags is very reliable. Like I'd bet my life on it reliable, even with hollow points. After my lower and mag swap, I wouldn't bet a dollar on its reliability. Does anyone have a blowback AR9 that uses the Mean Arms pmag conversions, and it's working reliably for you?

 

[Tombs]

Did this problem only start when you changed the lower to use a different magazine type?

Sounds like that system is intended for a specific barrel.

 

[92FSTech]

Are they so much better than standard blowback AR9s that the cost is justified, or is it mainly hype? Is the CMMG system pretty reliable?

Yes, they are that much better. I had a PSA blowback AR9 that ran Glock mags and it was an utter nightmare. My CMMG build with the RDB system on a standard Anderson lower with endomags has been flawless apart from one issue detailed below. It also shoots smoother and softer than the blowback gun, and has last round bolt hold open. Also, because it utilizes a rotating bolt like a standard AR, it has the same out of battery protection that's designed into a standard AR...the solid-bolt blowback guns ignore this in their design, and after you have a couple of rounds explode when not fully chambered and throw brass particles into your face (thankfully I'm not left handed!) you'll appreciate the need for that feature.

The only problem that I've had with the CMMG is that it will eat ejector springs after about 1000-1500 rounds. It will let you know when it's about to go, as ejection will get weaker and weaker before it stops ejecting completely. Supposedly since I bought mine, CMMG has redesigned the angle of the lugs on their bolt to correct this problem. I still need to get with them about a new bolt, but it hasn't been a huge priority. It's a 5 minute fix with an empty case, a punch, and a vice to replace a $1 ejector spring, so it's not a huge hardship.

Apart from that one issue it has been absolutely reliable, so I've just been treating it as a scheduled maintenance item.

ETA: one thing I forgot to add: CMMG recommends an enlarged ejection port for reliable operation. I used an Aero XL upper for my build for that reason.

 

[92FSTech]

Just because every thread needs pics:

 

[GunsNstuff]

Did this problem only start when you changed the lower to use a different magazine type?

Sounds like that system is intended for a specific barrel.

Yes, the problem started after replacing the billet Glock mag lower to the standard ar15 lower with the endo mags. I think the cartridge is held further back and at a different angle with the endo mags than it is with the Glock mags. Some 9mm barrels have a much larger feeding cone on the barrel, but mine is rather small. It's about the size of the Sub2000 feed cone. I'm using the same bolt as always, which is one made to work with both Glock and Colt magazines. I think I'll get the CMMG barrel and bolt, get my ar9 running again, then work on getting my current barrel and bolt running by testing out modifications to them.

One thing I do not like at all is that the ejector is only on the endo mags. Even though I know the mag has to be in the gun in order to eject the chambered round, I forgot once while messing with the constant jamming and when I pointed the gun downrange to drop the hammer to finish making sure the gun was safely unloaded, it fired when I did not expect it to. So give a point to proper safety habits. I assume the CMMG bolt will eject the chambered round even with the endo mag being out of the gun?

 

[92FSTech]

I assume the CMMG bolt will eject the chambered round even with the endo mag being out of the gun?

If you go with the CMMG bolt you will need to remove the ejector from your endomags or buy the version of the endomag that doesn't include it. The CMMG system uses the same pin-style ejector built into the bolt as a typical gas-operated AR.

 

[GunsNstuff]

If you go with the CMMG bolt you will need to remove the ejector from your endomags or buy the version of the endomag that doesn't include it. The CMMG system uses the same pin-style ejector built into the bolt as a typical gas-operated AR.

You were right. The CMMG RDB system is worth the money. Well, it's still expensive, but after shooting it, I am not upset at all that I spent the money. Swapping out the old 7.5 inch barrel, bolt & heavy buffer with the CMMG 8 inch barrel, BCG & normal AR15 buffer, the gun is much lighter weight with a lot less felt recoil. I cut the ejectors off my endomags, fired maybe 150 rounds of factory fmj, cast lead reloads and jhp reloads and it cycles everything. It's really nice and back to being my favorite gun again.

 

[92FSTech]

You were right. The CMMG RDB system is worth the money. Well, it's still expensive, but after shooting it, I am not upset at all that I spent the money. Swapping out the old 7.5 inch barrel, bolt & heavy buffer with the CMMG 8 inch barrel, BCG & normal AR15 buffer, the gun is much lighter weight with a lot less felt recoil. I cut the ejectors off my endomags, fired maybe 150 rounds of factory fmj, cast lead reloads and jhp reloads and it cycles everything. It's really nice and back to being my favorite gun again.

Glad it worked for you! I really think it's one of those things that you have to actually try for yourself to believe the difference...but the difference is immediately and clearly apparent. I love my 16" rifle, but long-term I'd love to put together an 8" SBR with a can and run subs though it. Just gotta deal with the paperwork, wait time, and $$$.

 

[GunsNstuff]

It functioned reliably with 115 and 124 grain. It functioned with the light weight AR15 standard buffer and also with the heavy AR9 buffer. I do want to SBR this gun so I think I am going to have to change the lower on it again, but it will be a standard AR15 lower. This particular lower is a little out of spec. The roll pin hole for the bolt catch was drilled a little too low so the bolt catch doesn't quite catch the bolt. That's a new problem I've never experienced before. A couple of stamps for an sbr and silencer will make this little gun perfect for me.

 

[92FSTech]

This particular lower is a little out of spec.

I hear ya. If I ever get around to doing an SBR build, I'm going to source a lower from a company with a solid reputation and vet it well. I've had pretty good luck overall with cheap Anderson lowers, which are typically the easiest to find locally. But I've also had a couple with some minor issues, and seen a few others with bigger issues. It would really suck to jump through all those hoops to stamp a receiver that's out of spec.