Radial Delayed Blowback

[Demi-human]

CMMG MKG = Glock mags , MK9 = colt/uzi style magazine. Which doesn't make any difference until your ordering a magazine. to misuse a model number in this conversation means nothing . Both work Great .

I'm drifting a bit, but it IS ONLY for background information NOT discussion,

Not at all, this is your thread, and I thank you for the information. I haven’t spent much time reading after finding the link, but with the power out, I’ll have time today.

I’m very excited to have another viable, more comfortable and less finicky, option to getting a 45 Auto running in an AR.

 

[Sgt_Mike1]

@Demi-human
look at this link

 

[someguy2800]

@Demi-human
look at this link

thats a great video. The first time I shot mine I couldn’t believe how soft shooting it was.

People never believe me when I say this but when I put together my first blowback AR9 the bolt velocity was so high that my pistol brace would actually bruise my shoulder from the buffer bottoming out. Switching to an AR10 extra power recoil spring and a buffer tube snubber helped, but nothing compares to a delayed blowback.

 

[Sgt_Mike1]

The more I look at this format of operation while discussing it. I'm really liking the concept. I didn't pick up a very subtle yet important aspect in the bolt /BCG until i seen the video is that the bolt is actually spring loaded. However when you go to the cmmg parts website in the RDB repair kit there sits the spring for the bolt.

The retail price is kind of steep to convert, but the weight saving coupled with the recoil impulse might be worth it.

@someguy2800 you are correct on the bolt velocity which is why at one point Colt went to a heavy hydraulic buffer to reduce the cyclic rate.

 

[greyling22]

The more I look at this format of operation while discussing it. I'm really liking the concept. I didn't pick up a very subtle yet important aspect in the bolt /BCG until i seen the video is that the bolt is actually spring loaded. However when you go to the cmmg parts website in the RDB repair kit there sits the spring for the bolt.

The retail price is kind of steep to convert, but the weight saving coupled with the recoil impulse might be worth it.

@someguy2800 you are correct on the bolt velocity which is why at one point Colt went to a heavy hydraulic buffer to reduce the cyclic rate.

Yes, it is spring loaded, so unlike a regular ar upper, the bolt will try and fall out of the upper if it is not attached to a lower.

 

[zaitcev]

The more I look at this format of operation while discussing it. I'm really liking the concept. I didn't pick up a very subtle yet important aspect in the bolt /BCG until i seen the video is that the bolt is actually spring loaded.

I'm not sure the spring-loaded bolt is a virtue of the RDB system. The reason it's necessary is that in a normal AR the flat side of the cam pin slides on a rail in the upper and guarantees that the bolt lugs are properly aligned with the barrel extension. But for whatever reason CMMG re-positioned and re-profiled the cam pin enough that it became impossible. So the spring extends the bolt and that makes its lugs align for entering the barrel extension. This solution potentially has upsides and downsides. It cannot be as reliable in presence of dirt, but then there's no gas inside the carrier and thus no carbon anyway. It also eliminates the rubbing, but then not many people wore out their rails in their ARs.

 

[TonyAngel]

I know this is an old thread, but I thought I'd post here since what I have to share jives with the topic.

I was running a blowback 9mm Ar for a while. I used a springco blue spring, a ballistic advantage bolt and a vltor A5SH4 buffer. My total reciprocating mass was something around 23 Oz. I really didn't like shooting it because as well as it shot, it seemed like a lot of work for what I was doing. Sending 115 to 124 gr pills down range at a rather sedate velocity.

I stumbled across a CMMG RDB barrel and bolt combo at sportsmansguide.com for something around $360 to my door and decided that wasn't bad for a barrel and bolt, so I grabbed it.

I used a run of the mill AR upper. You don't need to worry about ejection port size until you get bigger than 9mm. I used an H buffer and standard buffer spring. I used an H buffer because the lower is a host to a 5.56 upper as well and I like the little extra weight for reliability; although the 9mm rig ran fine with a standard buffer.

I tried 3 different magazine setups. An adapter for Colt mags, an adapter for glock mags and Endomag inserts. All required modification to work with the RDB rig. Not a big deal, essentially clearancing for the head of the bolt.

I found that the Endomags worked best reliably feeding the broadest range of different ammunition and shooting from weird positions. With the endomags, the rig cycles when on its right and left sides (as when shooting from under a car), but jams up when upside down.

As for the questions regarding the locking lugs ... although they are technically locking lugs, I don't think they actually "lock." What I mean by this is that if you put the bolt into battery with a case on the head and you push a cleaning rod down the barrel, you can push the bolt out of battery. In this case, I think it more accurate to call them "slow down" lugs.

In any case, it shoots like a champ. The recoil with the straight blowback setup was closer to that of a 7.62x39 than 5.56. With the RDB setup, the recoil is milder than that of my 12.5" 5.56.

 

[zaitcev]

I found that the Endomags worked best reliably feeding the broadest range of different ammunition and shooting from weird positions.

The Endomag is a secret sauce of modern AR9. It offers a gradual feed path that Glock magazine cannot match.

Because AR was designed around a long rifle cartridge, the distance between the bottom of the stripper rail (lowest lug in case of CMMG RDB) and the axis of the barrel is far greater than in a Glock. With a Glock magazine, you have to place the magazine very close to the breech. Otherwise, the catridge escapes the feed lips before it enters the chamber and has to fly purely by inertia, while pushed by the bolt. But if you do what needs to be done, there's basically no space for any reasonable feed ramp or angle. So, the catridge has to rotate very steeply up, then scrub against the bolt face hard as it straightens for feeding.

This is the experience of designing and building adapters for AR9 speaking. Nothing I could do touched Endomag's reliability.

People who cut own AR9 barrels and bolts tried to help, by adding hoods and ramps to the barrel. I suppose it was beneficial. Still, it was a workaround.

 

[TonyAngel]

I should also have mentioned that Mean Arms is getting ready to release the ExoMag. It is a complete magazine in a variety of capacities. Price is around $25.

 

[zaitcev]

I may be misremembering, but I'm pretty sure MEAN literally invented the Endomag, then something happened and it's primarily known as a CMMG schtick. I never heard of any acrimony between them, so it must've been an engineering contract, a licensing deal, or something of such nature. It's not surprising they came up with a follow-up.

 

[greyling22]

Exomag exists. https://www.meanarms.com/products/detail/exomag-20-round. They even have a 40rd mag. They are pretty janky looking though. I'm not a fan of the skeletonized look.

 

[TonyAngel]

Exomag exists. https://www.meanarms.com/products/detail/exomag-20-round. They even have a 40rd mag. They are pretty janky looking though. I'm not a fan of the skeletonized look.

The description indicates their being available as a pre-order. I'm gonna wait until they're actually available.

 

[greyling22]

The description indicates their being available as a pre-order. I'm gonna wait until they're actually available.

good catch. I was on my phone and totally missed it. apologies. I would be down for a 15 or 20 rd mag. I usually run 20's on my AR's. I like less mag hanging out, and I never load more than 10-15 rounds anyway. it slows my down and helps conserve ammo.