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CMMG RDB LWRC rifle


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I just finished testing a 9mm carbine I built using a 16" CMMG RDB 9mm kit.  All I can say is I really like this setup.  It shoots really softly and recovery is quick.  So far only put 100 rounds through this rifle with no issues

Specs:

LWRC old style rail with adaptor

Saber Defense upper receiver with no M4 ramps

Trijicon MRO

Radian Raptor charging handle

LWRC New style lower with Geissele SSA-E trigger.

 

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On 2/22/2020 at 9:27 AM, Joedirt199 said:

What weight buffer did you settle on? I am running an 8.5" barreled one and can still cycle ammo with an H buffer. Nice light recoil. Ejection is a little more forward but no stopages yet.

I am using a standard buffer.   I picked up a older Taccom PCC buffer. I' going to see how this feels 

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  • 2 weeks later...

I just built a CMMG RDB system with an 8" barrel and for now using it with an Aero mil spec lower and the CMMG 9 ARC PMags, as a pistol/other.  Waiting for barrel extensions to be back in stock at Taccom so that I can make 16" and run it as a main PCC (I didn't want the whole 16"+ barrel version, as I want to keep it lighter out front).  Only done a 250-rd test run so far with Blazer Brass 115 and 124.  Ran great, reliability with PMags has been great, and got lockback on empty mag each time.  Currently running it with an H1-equivalent Armaspec silent captured buffer system, looking to also try a standard Carbine 3 oz. buffer, and then see what a Blitzkrieg carbine/RB5000 hydraulic buffer will be like.

 

 The recoil impulse is noticeably a bit softer than my blowback PCC's.  But I have those tuned pretty well with a combination of hydraulic buffer/A5-tube with spacer/wave spring/Limbsaver Recoil Buttpad that at least for factory loads, it's what I consider an ideal combination of cycle speed/dot recovery and absorption of recoil.  If my blowback AR9's were setup more 'out-of-the-box' I'm sure there'd be much more difference.  But with the CMMG, just the setup mentioned is a little less kick but maybe a bit more dot bounce since it cycles slower.  I feel there is room for improvement, but I'm concerned that like with my regular blowback AR9's, whatever weight was added to buffer and cycle speed reduced, it made the reciprocating mass feel like it was 'lingering' more during the cycle.  I tend to not mind the kick as long as it's quick and your dot bounces right back.

 

I've read of someone getting what they felt was best results with an A5-length buffer  tube, a Blitzkrieg RB5007 (9mm/longer) hydraulic buffer, and a Tubbs flatware spring.  The 'Amphibian' (member on AR15.com) combo, I guess.  I want to try incorporating the wave spring that I have in my other PCC's, both of which have A5 buffer tubes, but for now the lower I'm using the 8" barrel on has a standard carbine tube.  Want to still leave room for bolt to move far enough back to lock, so I'll probably have to wait until I add barrel extension.  But I think there's great potential here to make an ideal PCC, at least for me.  Just want to make sure that I'll have enough confidence in the Pmags, which so far are doing great.

 

Sorry for such a long post, but I'm trying to detail things for comparison.

Edited by MoRivera
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Mo... my RDB system uses a rifle length buffer tube and OEM rifle spring... the buffer is a rifle length  Enidine from my 5.56 3 gun rifle, and I use a poly NFA bumper to shorten the overall bolt travel... after removing the extractor polymer plug and replacing with a Bravo Company HD extractor spring have about 700 rounds of 140power factor loads without a problem using ETS 40 rounders

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6 hours ago, Les Snyder said:

Mo... my RDB system uses a rifle length buffer tube and OEM rifle spring... the buffer is a rifle length  Enidine from my 5.56 3 gun rifle, and I use a poly NFA bumper to shorten the overall bolt travel... after removing the extractor polymer plug and replacing with a Bravo Company HD extractor spring have about 700 rounds of 140power factor loads without a problem using ETS 40 rounders

 

 

Nice.  Good thing is that my dedicated PCC lower (built on a pre-ban mil spec, since I'm in CT) already has an A5-length buffer tube.  So when the barrel extensions at Taccom are back in stock and I build the 16" upper, I'll have some leeway to find what I like best in terms of buffer/spring combo.  Right now, I'm just using a shorter upper on a pistol/'other' lower with brace.  Thing is that I want to do what I can to maintain LRBHO since I'm in a 10-rd mag limited state.

 

I also got some Spring enhanced springs for both the extractor (upgrade kit) and ejector.  How has your ejector spring held up?

Edited by MoRivera
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So I did some comparisons between three different buffer systems, shooting the same Blazer Brass 115 that I normally run through my AR9's.  a) Armaspec Silent Capture H1-equivalent, b) Armaspec Silent Capture H2-equivalent,  c) Blitzkrieg RB5000 (Carbine model) Hydraulic with xtra power carbine spring.

 

As far as buffer mass goes low to high, it's the Armaspec H1, the RB5000, then the Armaspec H2.  Not a huge swing, we're talking 3.8oz, 4.2oz, and 4.7oz.

 

 

Comparisons were pretty similar to what I had experienced with my regular blowback AR-9's.  I'll use the H1 as the base.  

 

H1 - a little sharp on the kick, but quick on dot recovery.  

RB5005 - softest and roundest on kick, more 'springy' feeling at the extreme ends of travel, more dot travel or maybe it seemed that way because it 'lingered' more

H2 - kind of best of both worlds above, softer kick than H1, quicker dot recovery than RB5000...BUT, like another shorter on YouTube, I got a failure to extract on a string of fast double-taps.

 

So I thought the H2 would probably be out, as it's most likely to jam.  But strangely, when I put the RB5000 back in, I started getting failures to extract towards the end of my session.  Maybe because of fouling, or because things started to wear at what was by now probably about 1200 rounds.  Checked the bolt, the ejector still seems to be fine (but I haven't pulled out the spring yet).  As far as just pure shooting, I think I preferred the H2 the most for obvious reasons.  But if it's more prone to FTE's than the other two, I wouldn't want to use it for competition.  So if choosing between what is left, I'd go with the H1 because I'll take a little more kick to the shoulder if the cycle and dot return is faster.

 

That's pretty much how I tuned my current competition AR9, through which I only really shoot factory loads like Blazer Brass.  So in comparing the two different guns, essentially the CMMG RDB system is a noticeably but not drastically more gentle version of the regular blowback AR9.  Now, those still aren't apples to apples, because my AR9 has a full-length MBX Ultralight barrel/comp which even though lightweight is still more mass out front compared to the 89" barrel and aluminum KVP Linear comp on the CMMG.  Also, my AR9 runs a hydraulic .308 buffer as well as a wave spring, AND a Limbsaver recoil pad so it's pretty tuned as far as factory loads go.

 

The CMMG along with the short barrel has a carbine-length gas tube and is mated to a mil spec lower (using the 9 ARC Magpul mags) with a SB-A4 brace which is stiffer on the shoulder.  So I have a feeling that if I ran the CMMG RDB system with a longer barrel with my regular soft-padded AR9 lower (preban mil spec with Stern Adapter), the difference would be even more noticeable.

 

So as it stands, I still need to get to the bottom of the FTE's at the end.  Maybe the buildup of fouling combined with the cushioned initial movement with the hydraulic buffer had something to do with it, since the CMMG bolt doesn't really fling out snap caps like other guns do when cycled dry...which makes me think it relies more on the velocity of the bolt moving back under fire.  But so far I'm most inclined to stay with the H1 Armaspec Captured if I were to shoot even this short version in a local competition.  Want to make sure before switching the upper with a longer extension to my main competition PCC ,so I still have some experimenting to do.

 

kOyJI8i.jpg

Edited by MoRivera
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After some more reading, it seems that 'amphibian' at AR15.com concurs with my assessment of initial bolt speed being key to ejection.  Les (sorry to keep asking you stuff), when cycling a snap cap or dummy round, how is manual ejection on yours?  Mine basically drops down through the magwell, but ejects during live fire.  About 2 o'clock and about 4 feet away.  I also removed the extractor plug and put in a Springco enhanced spring.

Edited by MoRivera
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Mo... when I hand cycle dummy rounds, they cleanly eject out of the port...in actual use about 5' at 4o"clock... I get a slight drag by the bolt lugs on the next to be cycled case, but after my initial attempts to modify the follower, have decided not to worry about them... I use short MagPul Glock mags for reloads that clear completely, and 40 round ETS to start a stage, now without any follower modifications...

 

amphibian has isolated most of his problems to weak ejector springs...I don't believe I had the same problem, as mine has cleared up with the removal of the plug in the extractor, and replacing with a new extractor spring ( I did not try the OEM spring after removing the plug).... additionally amphibian is shooting a full auto/supressed short barrel with factory ammo.... the cam slot determining the quickness of the bolt unlocking is quicker for the CMMG than a 5.56 AR, and that coupled with the short case of the 9mm vs. 5.56, to me anyway, makes me more suspicious of the extractor not snapping over the rim of a cartridge

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I just replaced my ejector spring as well.  The one that came out of the bolt didn't really look compressed or bent, but it was about 1mm shorter than the Springco replacement...that just may be differences in stock length.

 

It's weird when I manually cycle.  When I do it slow, I can see that the extractor claw has the rim, and the ejector is coming the case against the right side of the chamber, so you would think it's ready to fling it out.  But then it just sort of drops out as the bolt moves farther back.  Even when I do it quickly.  So that made me think that there's weak or no ejection.  But when firing it does come out.  Again not super far but not just trundling out either.  I have yet to live fire with new extractor and ejector springs, but just cycling dummy rounds it is the same as before.

 

I'll give a her a good cleaning and try again, I'm really hoping that once these ejection issues are solved I can stick with the H2 Armaspec buffer as it feels the best but also it's what I use with this same lower when I pair it with a 10.3"-barreled 5.56 that I shoot suppressed.  That way I don't even have to change buffers between the two.

 

Amphibian also mentioned that on some of his combos there was excessive headspace that he could check with a rod down the muzzle on a closed bolt.  Mine doesn't seem to have that as far as I can tell.  We'll see.  I'd really like to make this my main PCC upper (once I get a barrel extension).

Edited by MoRivera
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I was the person who recommended the insanely strong BCM extra power extractor spring to Les.
 

In my opinion that is a vital upgrade to these rifles.

 

I did replace my ejector spring with a factory power one at 3,000ish rounds when my problem began. It made no difference to ejection pattern or reliability. Changed the extractor spring to the BCM and I haven’t had a malfunction since.

 

Edited by MemphisMechanic
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Here is a comparison pic for what I just replaced....

 

kMNrLBa.jpg

 

Fb8Ov2Y.jpg

 

 

Big difference in extractor springs, not much in the ejector springs.  As you can see, after around 1100-1200 rounds the ejector spring doesn't seem compressed or bent, so maybe that's a more recent version.

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Memphis... I apologize that I did not identify you as the originator of the extractor spring upgrade, I will attempt to do so in the future... ... I have three matches and about 100 or so trial rounds without a problem since the upgrade...

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1 hour ago, Les Snyder said:

Memphis... I apologize that I did not identify you as the originator of the extractor spring upgrade, I will attempt to do so in the future... ... I have three matches and about 100 or so trial rounds without a problem since the upgrade...


No worries on crediting me. I chimed in on this thread to reinforce your suggestions on making Guard platforms run reliably and make sure their effectiveness was known.

 

I wasn’t worried about receiving credit. ;) 

Edited by MemphisMechanic
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2 hours ago, MemphisMechanic said:

That backs up my experience. I saw no difference visually with the ejector spring, and the gun ran just as poorly.

 

Extractor spring took care of every trace of my ejection problems.

That's good to know!  Thanks.  What buffer system are you running?

Edited by MoRivera
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Mo... I'm running a 5.56 Enidine rifle length hydraulic I pirated from the 3 gun rifle, factory rifle spring (I had previously used a .308 spring with the pre Cmmg blow back bolt, and had some battering to the rear of the boss area around the chamber with a 6.2oz rifle length buffer) both the original blow back and hydraulic with a NFA short bumper to shorten the stroke

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I've been following the threads on the RDB system with interest.

I put a PCC together with a Leadstar Arms skeletonized receiver set and a parts that have been around mostly from 3 gun prize tables. I'm running the 16" barrel, with an Ace stock on a rifle length tube, Hiper Fire 24C trigger. I lowered the mag catch about .045 thousandths so the mags cleared the bolt. My Glock mags, I removed the taper so the bolt cleared with no problems, then took the step off of the follower so nothing hung up. Much like Les did. Magpul mags feed fine in it. I did make a coupler or splice for them, which allows 51 rounds to be loaded. 

I haven't changed the extractor spring yet, but, did replace the ejector with a chrome silicone spring from Brownells. The ejector springs get compressed, and look like a worm after a short time, unlike the same spring in an AR with a centerfire round. I'll look at the extractor spring shortly. Honestly, I haven't shot the gun a lot. 

I'm running a standard rifle buffer and spring at this time with no modifications. A Tubb flat spring is on the list to try when I get to it.

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