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Posted: 11/10/2010 5:11:16 PM EDT
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Hey all,
I've done lots of reading on the short stroking situation and now need to get help for my specific problem: Bolt fails to lock back after last round fired. This happens about 80-90% of the time and had been ongoing since I built up the rifle. It happens no-matter what mags I use. Sometimes it locks back on particular mags, sometimes not. Other than this problem the gun is accurate and performs well. These mags all perform great in my other AR. I have built the rifle using the upper and lower (including internal parts) from a Colt A2 HBAR. The 20" barrel and and butt from the HBAR went to my other rifle. I posted this topic several months ago and some people were asking following info, like bolt weight, buffer weight, that's why I include it. I have now taken apart the rifle to get a look at the components most often mentioned in the forum and hopefully have all the info, if not just let me know what other things you may need to help: 1. Rifle only has approx 100 rounds through it. 2. Barrel is 16" with gas hole 8 3/8" from muzzle 3. Rifle has collapsable stock. I originally purchased a carbine buffer assembly for the build. Buffer weighs 84g (2.9oz) 4. I then purchased a Advanced Armament Corp Cyclic rate reducer CSMKII to help reduce recoil. Weighs 151g (5.3 oz) Short stroking happens with either buffer. 5. Removed bolt carrier and torque checked the key bolts. They torque checked fine to 40" lbs, and are well staked. 6. Bolt and carrier weigh 297g (10.45 oz). My second chromed bolt and carrier weigh 316g (11.15oz) 7. Tested the bolt gas rings by extending bolt and balancing bolt and carrier on table. Did not collapse. 8. Removed front sight and took a look at and measured gas port. I do have a digital caliper/micrometer and the hole measures at approximately .063" 9. The gas hole looks like it may have some gas leakage around it. I'm assuming this is the case because it looks like a carbon "star" shape around the hole. Is this normal? 10. I have 3 buffer springs that I can try in the rifle (4 if I include the one from the other 20" rifle). Is there a special length carbine spring? Never considered this when I built the shorty. One of the 3 springs is about 1/2 longer than the other 2. 11. The bolt catch seems to work fine when I insert an empty mag Any help that anyone could provide would be appreciated. |
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Given the info posted, I would focus on the buffer/spring. Just install a standard carbine buffer and spring.
Next I'd focus on the gas hole. #1 it seems to be leaking. That can't help things. #2 check with a builder to see what size it should be and make sure you are GTG. I have a 12.3" 6.8 SPC that had the same problem (I think) in that it was short-stroking. I opened up the hole just a wee bit and now it runs fine. I am not an expert by any means; however it sounds to me to be undergassed. Someone smarter than I shall be along shortly. Please enjoy the music . . . . |
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I would agree that the first item of business is to make sure you have a correct carbine length spring. A spring from a fixed stock/rifle length tube will not work correctly and will cause short stroking when used in the shorter carbine buffer tube.
The ring of carbon around the gas port on the barrel is normal after you pull off the FSB/gas block, as the hole in the gas block is larger than the gas port diameter. Unless you see evidence of fouling coming out from beneath the gas block, or a lot out on the gas tube where it exits the gas block, you should be fine. If you're swapped BCGs then gas key leakage shouldn't be an issue. Extreme cyclic rate/bolt velocity can sometimes mimic the symptoms of short stroking, but if you're tried a rate reducing buffer that should have answered that question. |
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Need barrel length and OD at gas port so I can tell if the gas port is correct, and the free length of the recoil springs.
Gas port for a standard carbine barrel is .063 to .078, Carbine recoils spring is about 10.5" long, while a full stock rifle recoils spring is about 11.75". |
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Just checked based on all the comments:
1. The original Colt A2 front sight gas block was removed off the 20" barrel and placed on this new 16" barrle for the build. 2. 4 springs measure as follows: 2 measure 11", one measures 11 3/4", and the one in my 2nd AR measures 12" I believe the 11 3/4 was installed in the rifle. This may be an odd question but can you trim the buffer spring down to length? 3. When dis-assembling yesterday I noted the gas tube was very difficult to remove from the gas block. I had to spray it down and leave it for a bit so I didn't damage it when removing from the gas block. The tube did have a bit of crud on it, extending about 1/4" backwards from where it enters the gas block. There was no leakage around the gas block onto the barrel. 4. The barrel is 16", with a gas hole of .063 diam Thanks for the responses. I may get the gun back together tonigh and head to the range tomorrow if weather stays nice. |
| I had to redrill my gas hole to .075 on one gun I built to solve the same problem....If you redrill only go up one size at a time like from .065 to .070 and test then .075 or if you have smaller steps it wise to drill out the smallest sizes possible since you can not add if you take it off..... Im lying you can but its very hard and you have to be a good welder to do so..... |
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Quoted:
Just checked based on all the comments: 1. The original Colt A2 front sight gas block was removed off the 20" barrel and placed on this new 16" barrle for the build.  Pinned on FSBs are not generally interchangable. It's possible that the FSB is partially obstructing the gas port and reducing the amount of gas flow. IF you have a full circle of gas residue around the gas port when you disassemble it then you should be ok though.
2. 4 springs measure as follows: 2 measure 11", one measures 11 3/4", and the one in my 2nd AR measures 12" I believe the 11 3/4 was installed in the rifle. This may be an odd question but can you trim the buffer spring down to length? The overall length is just a rough guide, what really matters is the number of turns on the spring. Rifle springs when fully compressed are just too long to allow the BCG to move far enough to the rear when used in a carbine length tube. Cutting them down is not recommended. 3. When dis-assembling yesterday I noted the gas tube was very difficult to remove from the gas block. I had to spray it down and leave it for a bit so I didn't damage it when removing from the gas block. The tube did have a bit of crud on it, extending about 1/4" backwards from where it enters the gas block. There was no leakage around the gas block onto the barrel. Completely normal. After a little bit of shooting the residue will (and should) seal the gas tube into the gas block. If you want to make it easier to remove in the future, put a coat of moly grease on the outside of the gas tube before installation. 4. The barrel is 16", with a gas hole of .063 diam Thanks for the responses. I may get the gun back together tonigh and head to the range tomorrow if weather stays nice. |
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Lots of good info from everyone.
Gamma762 1. Did not know that about the FSBs. I bought the 16" barrel on it's own. Should I attempt placing the FSB back on the 20" where it came from, then taking the one off the 20" and putting on the 16"? Easy enough job. 2. I'll try the 2 shortest springs in the gun and see what happens before ordering a new one. Sometimes it's hard to get parts in Canada or it takes a while to ship (weeks to months) 3. I've got moly and will try that tip for the tube installation. The carbon around the hole looks fairly uniform around it. I'll try all these fixes prior to drilling out the hole. I have access to a good machine shop at work and they have all necessary bits for the job. I think I have the bits at home but am a little hesitant to drill the barrel. After drilling, is there normally burrs on the inside of the barrel? Will these get smoothed out with shooting if it happens? Thanks everyone |
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Quoted:
I had to redrill my gas hole to .075 on one gun I built to solve the same problem....If you redrill only go up one size at a time like from .065 to .070 and test then .075 or if you have smaller steps it wise to drill out the smallest sizes possible since you can not add if you take it off..... Im lying you can but its very hard and you have to be a good welder to do so..... Good advice if you have to drill. On a SBR custom length, I went from .098 to .1065 - not much. but runs great, where it short-stroked before |
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Quoted:
Lots of good info from everyone. Gamma762 1. Did not know that about the FSBs. I bought the 16" barrel on it's own. Should I attempt placing the FSB back on the 20" where it came from, then taking the one off the 20" and putting on the 16"? Easy enough job. 2. I'll try the 2 shortest springs in the gun and see what happens before ordering a new one. Sometimes it's hard to get parts in Canada or it takes a while to ship (weeks to months) 3. I've got moly and will try that tip for the tube installation. The carbon around the hole looks fairly uniform around it. I'll try all these fixes prior to drilling out the hole. I have access to a good machine shop at work and they have all necessary bits for the job. I think I have the bits at home but am a little hesitant to drill the barrel. After drilling, is there normally burrs on the inside of the barrel? Will these get smoothed out with shooting if it happens? Thanks everyone Burrs: I put a wooden dowel inside as close to the diameter as possible to reduce them, ran some IOSSO or bore polish down post drilling using a mop, cleared it all using compressed air. |
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Well,
Took the rifleout last week and zero'd it. I'll post info and pictures elsewhere. Had to mention that this rifle was also possibly having the short stroke problem. Now I am at a loss. I think before I start drilling the barrel on both guns I am going to purchase 2 or 3 new mags. All the mags I use are fairly well used and showng good wear. The springs do seem to be working well but you never know. I am doing a trip to Ontario and there are better stores there to purchase kit. I'll pick up some mags, come home, re-shoot the 20" AR and put the 16" back together and try it with the new mags as well. Hopefully that will work. |
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You are saying "short stroking", but other than not locking back on an empty mag what exactly is happening?
The 20" barrel rifle, is that using a fixed stock with rifle length buffer tube or carbine telestock buffer tube? Did you ever get a known correct carbine spring for your telescoping stock setup? Best way to tell is to just assemble it and pull the bolt as far to the rear as you can, and see what the distance is between the bolt face and the bolt catch. With multiple rifles having problems it sounds like an ammo problem, but need to rule out other issues. |
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Dude, make sure you have the correct buffer and spring for the type of stock you have. Then pull the bolt back by hand on the assembled rifle the bolt should stop about 1/4 inch before the back of the eject port. Does it? That should give it room to lock the bolt back on the bolt catch on the bolt face. Now with the bolt locked back how much coverage do you have of the bolt catch covering the bolt? It should cover the bottom lugs pretty well. If not this could be a problem.
Now, with the lower only.....insert a mag gently. Does the bolt catch pop up easily and fully and completely. If not that is a problem. Also you are in Canada. Are you using some shit 5 round mags? That could be the problem. Can you get a 30 round usgi mag that has legally been blocked to 5rounds? Give that a try. Ps. Is the fail to lock on last round the only issue? If so don't drill anything yet. pps. I went to MtA |
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