300 blackout cycling issue????

Dustyj357

Dustyj357

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Full Member
Minuteman
Oct 29, 2010
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Waxahachie, TX
#1
Ok here's what I got:
16" CMMG upper
RRA lower
220gr Remington Match
Standard buffer/spring
Gemtech HVT threaded

Round fires, ejects, but doesn't grab the next round.

I tried removing buffer spring weights but that wasnt effective. What's next?
 
#4
Re: 300 blackout cycling issue????

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: *coyotekiller*</div><div class="ubbcode-body"><span style="font-weight: bold">Carbine.</span> What is the appropriate way to check alignment? </div></div>

That's likely your problem. For some reason the 300BLK doesn't do well in this length with a carbine gas system. If you get the pistol length gas system it would likely cycle fine. I've seen the 16" CMMG barrels in carbine length gas systems not feed properly on supersonic ammo and fail entirely on subsonic ammo (which it looks like you're running). Try supersonic ammo and my guess is you'll have slightly better luck but still not consistent reliability.
 
#5
Re: 300 blackout cycling issue????

Short stroking, like said above eather the gas block isn't lined up or the gas port needs opening up alittle. I have a noveske 16" with carbine gas, but it cycles fine with both super and subs, suppressed or not. Your just not getting enuff gas to cycle properly.
 
#8
Re: 300 blackout cycling issue????

Have you tried contacting CMMG?

I wouldn't be so quick to start drilling out the gas port, even if that is the problem. You shouldn't have to buy an upper receiver and then fix it, assuming that this is a complete upper receiver built by CMMG.
 
#9
Re: 300 blackout cycling issue????

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jakhamr81</div><div class="ubbcode-body">Have you tried contacting CMMG?

I wouldn't be so quick to start drilling out the gas port, even if that is the problem. You shouldn't have to buy an upper receiver and then fix it, assuming that this is a complete upper receiver built by CMMG. </div></div>


Very true!
 
#11
Re: 300 blackout cycling issue????

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Dr. Phil</div><div class="ubbcode-body">You will likely have to have the gas port enlarged.</div></div>
^^^ This. For subsonic ammo, there just isn't enough port pressure to cycle the carbine length system. To reliably cycle the subs, you will most likely need to open the gas port and reduce the buffer weight. This will get you there.

Also, I would recommend trying my subsonic ammo. I designed this ammo to help with this problem. It helped a few AR's that had this issue. The problem with most factory subs is they use a very light powder charge with a fast burning powder. By the time the bullet passes the port, there is not enough pressure built up to cycle the rifle. I designed the ammo with a specific powder to provide a high charge which gives it more port pressure. I'm not saying it will solve your issue, but it could.
 
#12
Re: 300 blackout cycling issue????

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Dr. Phil</div><div class="ubbcode-body">You will likely have to have the gas port enlarged.
The issue you describe is pretty common with the CMMG 300 BLK barrels.
Check out http://www.300blktalk.com/forum/

Sorry...
I know that was not what you wanted to hear. </div></div>

This is exactly what I wanted to hear. I was curious if it was an issue with the CMMG
 
#13
Re: 300 blackout cycling issue????

I recommend first to try slower powder, I had the exact same problems with mine. I first tried opening gas port, still wasn't perfect, switched to vihtavouri 530 powder, and works great. I was even able to switch back to heavy buffer and m16 spring. Try slower powder first, (530, h322) see if that helps, if not then try opening gas port.
 
#14
Re: 300 blackout cycling issue????

CMMG seems to have designed these barrels to be supersonic only. That is no inherent reason why a 16 inch barrel with carbine gas cannot work well with subsonic - AAC and Noveske, after all, use that configuration with no problems. One should use a "C" buffer though (not an H or H2 buffer).
 
#15
Re: 300 blackout cycling issue????

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rsilvers</div><div class="ubbcode-body">CMMG seems to have designed these barrels to be supersonic only. That is no inherent reason why a 16 inch barrel with carbine gas cannot work well with subsonic - AAC and Noveske, after all, use that configuration with no problems. One should use a "C" buffer though (not an H or H2 buffer). </div></div>
LOL.
How do you "design" a barrel to be supersonic only? Kind of sounds like an engineers term.
Being an engineer for about 25 years I can say we designed and engineered buildings but I can't say I have ever designed a hole.
Sounds to me like someone "decided" to drill the gas port a fuzz small.
OP,
In order not to over-gas the barrel with supersonic loads the barrels normally will not lock back shooting subsonic loads without a can. the port was either .106 or .110. If you want it to lock back without a can it could be over-gassed when shooting supersonic. A paladin 3 way gas block could solve that problem though.
 
#16
Re: 300 blackout cycling issue????

I'm running a CMMG Barrel with Carbine Length Gas tube with no issues running Subs (with Suppressor) or Supersonics. If you want to do both, get the Pistol Length and an Adjustable Gas Block.

Otherwise, you may need to open up your gas port and get a Adjustable Gas block on your Carbine Length barrel if you want to shoot Subs without a suppressor...not sure why you'd want to do this.

Getting an AR to cycle correctly has to do with the gas system. You need to have the correct amount of gas without being over gassed. Sure you could swap out buffers, buffer springs, carriers, etc. but getting a Adjustable gas block is all you really need.

I recently installed a Syrac Adjust gas block. Syrac is different than the other blocks because their gas block adjust from the front and has a detent to prevent rotation after being set. I originally had a PRI Adjustable Gas block but I didn't want to drill my forearm so I could reach the adjustment screw.

Syrac_Gas_Block_Header.jpg


 
#17
Re: 300 blackout cycling issue????

Bustin quote:
In order not to over-gas the barrel with supersonic loads the barrels normally will not lock back shooting subsonic loads without a can. the port was either .106 or .110. If you want it to lock back without a can it could be over-gassed when shooting supersonic. A paladin 3 way gas block could solve that problem though.


That's the thing, I am using a can and it's not cycling. I could careless if it cycles subs unsurpressed. It seems like I will have to drill the gas port. Does the adjustable gas block bypass this step?
 
#18
Re: 300 blackout cycling issue????

I would start off with making sure your Gas Block is properly aligned with the hole in the barrel. Once you've ruled that out I would then make sure the hole in the Gas block is larger the the hole in the barrel.

If neither of those things work, then you could open up the gas hole in your barrel and install an Adjustable gas block so you could later turn down the gas if you drilled your hole too large or shoot supers.
 
#20
Re: 300 blackout cycling issue????

just go slow and make sure not to hit the other side of the bore.
I just checked the wall chart in the shop, we used a 7/64(109) the next step up would be a #34 or .111
 
#21
Re: 300 blackout cycling issue????

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bustin</div><div class="ubbcode-body">How do you "design" a barrel to be supersonic only? Kind of sounds like an engineers term.</div></div>

I can think of a few ways ranging from using too slow a twist to using too small a gas port.
 
#22
Re: 300 blackout cycling issue????

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: *coyotekiller*</div><div class="ubbcode-body">Alignment looked fine, best I could tell. Now on drilling, is that an easy enough DIY project or should I take it to an expert? </div></div>

Take it to an expert, runing a bit too deep will FUBAR your lands and grooves.
 
#23
Re: 300 blackout cycling issue????

Ok. In case of further inquiry it seems the buffer tube on RRAs rifle has grooves which cause friction. This friction sucks for subs. Had to replace the buffer tube Plus a lot of oil Was required on the bolt. Hope this helps others out.
 
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