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Posted: 3/26/2007 4:55:33 PM EDT
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Hey guys, I have recently built up a RRA 10.5", 1/9 twist, SBR. The lower is a factory RRA National Match. I have also added a Noveske Flaming Pig muzzlebreak. For some reason or another the darn thing just won't run right. I tried different ammo, different mags, etc. and have no idea where the problem lies. When it fires a round it will not eject the spent casing before trying to chamber a new round. I have done some troubleshooting on ARs before and always got the problem figured out, but this has me stumped. Any ideas on things to change out, switch around, or do differently? Any and all help would be appreciated. This was my first SBR, and if it doesn't get any better than this I will end up regretting the decision. Thanks again, Clint |
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(disclaimer) no expert(disclaimer) I'm sure that the real experts will be along shortly but until they do could you tell us more about what buffer and BCG your running? Is your carrier properly staked etc. I know these questions almost always get asked along the way. |
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buckfever34, I am by no means any sort of expert here either. My 7.5" would not eject the spent casing and would then try to shove both the spent round and a fresh round into the chamber. Sounds like your having the same problem. I couldn't find a solution and shipped it back to CMMG. They returned it with a new buffer and it has run fine ever since. You'll find a solution and won't regret your decision. Best of luck. |
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The upper that I have is a complete setup from J&T Distributing/Doublestar. The B/BC/CH is the one they send with their factory uppers. I have a VLTOR stock on the lower with a standard carbine buffer/spring. Let me know if any other information is required. Thanks again, guys. |
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I know when I got mine, I was TOLD I needed to run an H2 buffer in it to make it reliable, so I do. Mine is set up exactly like yours except an LMT upper and CTR stock. I have had absolutely ZERO problems with it, and the above post speaks about changing the buffer as well. Not exactly sure How the buffer affects ejection, but someone will be along shortly who will iron it out for you. Good luck. I love mine and I promise you once you get it running right, you will love yours. |
This is what I was fishing for in my post. I've been told you need to run a heavier buffer to slow down the bolt so the casing has time to clear. I'm sure someone else can state that in more precise words but that's my take on it. I know alot of folks run a 9mm buffer that is heavier than a standard unit. I run a 7.5" SBR with a standard buffer, but I'm also running an M16 carrier which has a little bit more mass. I'm not sure exactly how much over a standard carrier it has but it does run 100% with everything I feed it. When I built it I assumed it would not run correctly without a heavier buffer but for whatever reason it does. I'd drop 25$ on a 9mm buffer and see how it does. Good luck and don't get discouraged, there are plenty of SBR's that run fine, it's just a matter of getting it set up correctly. |
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+1 on the heavier buffer. SBRs with the carbine or pistol gas system bleed gas from the barrel at a higher pressure than with midlength or rifle length guns, and so the bolt velocity is significantly higher. Increasing the reciprocating mass will slow the bolt down; a 9mm buffer is a good starting point. The key thing is to observe the ejection pattern; with the light CAR buffer, the cases will usually eject forwards (like 1-2 o'clock, if the muzzle is 12), indicating that the bolt has hit the back of its cycle and is moving forwards before the case ejects. As you increase the buffer weight, the ejection will be more out to the side (3-4 o'clock) - this is ideal IMHO. If the 9mm buffer still does not do it for you, you can look at the MGI buffer - I've see them work miracles on otherwise unreliable guns, but be prepared for sticker shock. Another alternative is to install an adjustable gas tube (MGI make these too), and this can be a better option if you want to shoot fast and accurately (like in 3-gun competition)... the heavier buffers will all increase the sight picture disruption due to recoil. |
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BEFORE you drop $100 on a new buffer try this first. www.ar15.com/forums/topic.html?b=7&f=25&t=424633 May be the best $5 you'll spend. I had issues with a 7.5" upper and upgraded the extractor spring and did not have any problems after that. Oh yeah, I have a 10.25" RRA upper and it runs smooth as silk. I also have the upgraded spring extractor kit, but standard buffer/spring (actually I have a substandard buffer/spring because it's a no name brand that I picked up at a gun show).  
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| Honestly you need to sit back and think about what is happening. When you think about it...how would a buffer really affect the carrier losing it's grasp of the rim of the cartridge. Dont get me wrong, it will affect extraction to a certain extent but it sounds like you have other problems. Carbines eject very violently. Shorties even more so. Seriously give the extractor upgrade a try. I've always had good luck with dfender d rings. Yeah they cost a lot for what they are, but they are supposed to add a 300% increase of tension over the stock extractor insert and spring. If your bolt is cycling so fast that it cannot grip the rim and hold on well enough it sounds like an extractor issue to me. |
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The MGI buffer, though expensive, is an amazing piece of gadgetry. It's the heaviest buffer out there, and has two tungsten rods on the rear plunger protruding back into the buffer itself. Not only does it slow the cycling from it's weight (7 oz? someone correct if I'm mistaken) but since the plunger hits the back of the tube first the rods smack the buffer as it cycles back - further negating felt recoil. Pretty cool addition. I think the cost is $175. |
When you shoot, the brass case swells to seal the chamber then slightly shrinks for extraction. Too light of a buffer and the bolt is trying to extract the case while it is still expanded in the chamber, the extra friction causes the extractor to slip off of the case rim, or even rip part of the case rim off. If all you do is upgrade the extractor spring, you'll start ripping case rims off if your real problem is being under buffered. A heavier buffer keeps the bolt in battery just a fraction longer, allowing the case to shrink before the extraction process. I'd go to a H2 buffer and upgrade the extractor. |
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Ok guys, I got a chance to go out yesterday and try it out with a different B/BC. With a different b/bc out of another gun it worked flawlessly. I tried the bolt that was given me problems with another gun and it worked just fine. I have ordered extractor upgrades for all my ARs  Thanks for all the help. |
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Huge sigh of relief huh. I have a 7" upper that I just sent back to PK for testing. I couldnt get it to run either. I was not having extraction problems. Mine wouldnt strip another round from the mag once you fired. I also tried 3 types of ammo and 3 different types of mags. Parts from a different rifle as well. Paul at PK was very understanding of my issue. Hope to get it back soon. I have one of the MedCon 10" uppers from CMMG on the way. I hope it runs. Congrats on your 1st SBR!!!!!! |
Be very careful not to use a bolt from another weapon in your weapon. I'm glad you found out that your issues could be fixed with the recommended upgraded extractor spring... and not blow yourself up. Now go get yourself that upgraded $5.00 spring and enjoy burning powder! 
This goes for you too. Link I was lucky to find out early why my 7.5 inch upper wouldn't run. After I upgraded the extractor sping, it ran like a champ. Everything else you buy susbsequently (heavier buffer, KX3, etc...) is just gravy and piece of mind, but the extractor spring is usually the main culprit. Try it first. It's cheap and easy and best of all, it works!
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