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Posted: 5/21/2006 11:23:04 AM EDT
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OK guys, I am stumped. A buddies AR is having FTF short stroke problems after about two mags. Every time we pull the bolt out after shooting, at least two of the gas ring gaps are lined up. This is a fairly new (800 rounds or so) CMT bolt. I just switched the gas rings, but we have not test fired it yet. It does it with either a CMT BC or a Colt BC. I have built dozens of these things from boxes of parts over the years, but this one has me pissed because I can't solve the riddle. Has anyone ever seen this?? thx |
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Multiple quality mags with Magpul followers and ISMI springs. It also does it with 20 rounders. It does it with the upper on my lower which works fine and has the same set-up. It ran fine for hundreds of rounds. The upper was completely taken apart and put back together to check everything. The only part not swapped out for a test was the bolt. Really, really weird. The one common thread is that the gas rings line up on this bolt after firing. I have never seen that before. |
| While I don't know a damn thing I would look a bit closer to the source of the problem. What I am sayin is that the groove where the gas rings go might have a little baby burr internal which is snaggin the rings as they go around and around their merry way. This is more likely than some rifle has put a curse on your rifle.... Give a look! |
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The gas rings are in motion when the rifle is fired, and at any give time, they can be aligned or out of alignment. As stated above, the alignment of the gas rings has nothing to do if the upper is short stroking or not. __________________________________________ As for the original question/post, and myself, I'm still waiting for the specifics on the rifle/build before chiming in. As of now, the best I could do is beat the walls with the Seeing Eye cane until more information is offered up. Until then, you will find it best to go threw the FAQ's. Granted that you may not find the specific answer you are looking for right out, at least you will have an idea of how to ask the question so we can trouble shoot the rifle. 
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Me thinks 308Sako may be onto something with the bolt, and I can't believe I forgot to look. Please note in the original post, TWO different Bolt Carriers were used in a series of single variable experiments. My buddy and I have two identically built AR'S (out of many), mine has been 100% for over a thousand rounds, his was 100% for the first 750 or so. At the end of the 200 round session that culminated in the 750 round count, the gun began to short cycle. We disassemble the gun, noting that the gas rings are aligned, but seeing no other obvious gas system issues. The gun was then cleaned and the next session resulted in short strokes after 100 rounds, with the gas rings found to be aligned again. Offsetting the ring gaps allowed us to shoot aother 50 or so rounds before the short stroke problem reappeared. We then detail stripped the upper. the barrel was confirmed to be adequately torqued, the front site base was properly aligned with the gas port as evidenced by the carbon trace around the port, the port, FSB and gas tube were found to be free of burrs or other blockages. Everything was beautifully aligned from front to rear, with a bolt carrier sans bolt freely slipping over the gas tube as before. The gun was reassembled and a different bolt carrier was installed, this resulted in short strokes after 50 some rounds and of course the rings lined up again. A few additional observations - The problem is getting worse. Swapping lowers made no difference to the malfunction. My lower with his upper barfs, but purrs with my upper. Multiple new mags make no difference. Swapping bolt carriers (Stag/Colt) made no difference, eliminating gas key issues, etc. WW White box M193 and XM 193 = no difference. Mine will even run on the anemic American Eagle and lock the bolt open evey time. The only remaining assembly that has not been changed out in an experiment is the bolt. The bolt will get replaced Friday. Note: the guns are 16" mid-length gas systems. Larue rails on STAG M4 cut uppers. The bolts are Stag/CMT as are the carriers. The lowers are Stag, with Geissele triggers, rifle length buffer tubes with rifle buffers and ISMI springs. Mine runs like a champ, my buddy got the problem child. For reference, we also run rifle buffers/tubes and ISMI springs on a number of 16" carbine length gas system guns with stellar results over thousands of rounds. |
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A bolt carrier assembly obviously has mutiple functions, but can still be thought of as a singular sub-component. If changing a major sub-component results in a measurable change in the response, further investigation into the functions impacted by said sub-component would be certainly indicated. But, it is quite likely that if we isolate it down to one item like a bolt, that while really nailing down the reason for the problem would be satisfying from an academic standpoint, it might be more satisfying to see how far we could throw the frickin thing. |
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Looks like the problem is in the upper receiver (carrier track too tight), the chamber (it/extension rough or on the short headspace side), or the gas system on the problem upper (barrel getting hot and leaking out at the FSB). Don't be too quick to jump to the gas rings as the problem. When you pull a B/C, and reinsert it, the existing lube has time to migrate in the upper and even on the B/C it's self, and can partial self correct a fouling, binding, or rough chamber conditions. Have you tried just pulling the B/C when the rifle starts to jam and re-lubing the B/C with CLP (read spraying a heavy coat inside and out, then giving it a quick shake before putting it back into the rifle), and the new lube did not resolve the problem? |
 Perhaps it's a solution without a problem, but for many, reassembling an AR-15 involves aligning the gaps in the gas rings to make sure they are as far apart as possible. We do this to avoid all sorts of problems and malfunctions. Finally, there's a solution that eliminates the need to adjust the rings' alignment, and lessens the chance of jamming.How It's Supposed To Work Upon firing, as the pressure of the gas generated by the burning propellant drives the projectile down the barrel and past the gas port, a small quantity of the gas is bled off through the gas port, gas tube, and bolt carrier key into the cylindrical section in the bolt carrier where it expands and drives the bolt carrier rearward. During the first rearward travel of the carrier, the bolt is rotated by the cam pin acted on by the bolt carrier cam slot. This rotation disengages the bolt lugs from the barrel extension lugs and so the bolt is unlocked. The carrier then continues rearward with the unlocked bolt. At this point, the gas used to drive the bolt carrier rearward is allowed to bleed out through two holes on the right of the bolt carrier. The Problem In order to install the rings on the bolt, they must be split and thus a "gap" on each ring is unavoidable. The problem arises because these gaps can become aligned, and cause too much gas to escape too early in the cycle. This can result in short-stroking and possibly jamming the rifle, so manuals and instructors enforce the proper alignment of the rings when reassembling the AR-15 rifle. The Solution The McFarland one-piece gas rings solves the problem of "aligned" gaps by eliminating the gaps. As a one-piece helical ring, you are guaranteed to never have the problem with the gaps. Replacement The 3 individual rings can be removed one at a time starting with the rearmost ring first. Lift one end of the open ring up and over the edge of the ring groove (towards the rear of the bolt) and then work the other end over. Repeat this for the remaining two rings, and you should be able to remove them without damage. As a single piece of metal, the McFarland ring is wound onto the groove on the bolt's rear. Start one end over the edge, and then work the remainder of the ring over that edge; the easiest way to accomplish this is without trying to turn the ring itself. Ordering If you're interested in trying these out, the McFarland rings can be ordered from Competition Specialties and are $1.75 for 1 - 4 quantities, $1.50 for 5-10 quantities, and $1.25 for 13 and up quantities. The shipping is included in the price. These prices are for the US sales only, outside the US you must call for shipping quote. At those prices, every AR-15 should have the one-piece ring installed, and it's owner should keep a couple of spares around. While I have never experienced the dreaded "gap-alignment" syndrome, the theory appears sound and a little extra insurance can't hurt (at least I no longer have to worry about aligning, or mis-aligning, the rings each time I assemble the rifle!). For more information or to place an order check out: Competition Specialties 105 E. Cass, PO Box 451 Osceola, Iowa 50213 515-342-2011 800-369-4481 [email protected] |
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I have often considered trying the McFarland ring, however, one advantage of the multi-piece piston ring system is that the ring on the low pressure side can sit flat against the gland wall to minimize leakage under the rings. This particular bolt/ring assembly is the only one I have ever seen that aligns its rings consistently. Just for reference, one of the types of products we produce where I work are metallic seals. A large number are used in turbine engines as outer air seals between compressor stages. In some applications there is not enough room in the seal cavity to allow for the differential expansion between either different alloys (seal to cavity components) or components with wildly different levels of thermal inertia (thin sheet metal seal to cast structure). In those applications we sometimes split the seal (like the gap in an AR gas ring). The leakage rate for these seals is an order of magnitude greater than for ones without the split. This makes me a firm believer in staggering the ring gaps on installation, and why I think it is bad when they line up. |
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New bolt.... no lining up of the gaps. Problem still exists. This is not a gas problem. Bolt and carrier assembly move freely back and forth, but....... Pulling the upper and lower halves apart and the bc out and taking a look with a flashlight reveals that the sides of the gas key track are now bare aluminum, that and the dry film lube is apparently missing along the contact points for the raised guides on the bolt carrier. This is a CMT, M4 cut upper. The Bolt carrier and key slide freely by hand and there is no apparent binding, but the hard anodizing is chipping off along the track. I read something a while back about an anodizing (adherence of) problem with Stag/CMT. Is this what that looked like? Also, is it common for the dry film lube to be gone where the carrier normally rides? My Colts look like new after a jillion rounds. My similar build has the dry film worn off wear the carrier rides, but exhibits no loss of the hard anodizing. Dano523, you had this nailed. Unfortunately, we did not look at it at the range, so no experiment with adding lube was performed. However, in looking for a smoking gun (pun intentional) that would get worse as the gun was shot, a nice area of galled up Aluminum would have to rank right up there as something that would act up as it got dry. The thing that is puzzling is how smooth it feels when hand cycling it. I guess at speed, it could be bouncing off the sides. |
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You have to remember that the only thing that keeps the bolt from spinning in the upper receiver is the key against the upper track, and then the cam down the same track once the bolt unlocks. Dry cycling the B/C will tell you if you have binding right off the bat, but you have to remember when the bolt gets unlocked, it's not you pulling back on the charging handle, its the gas pressure pushing back on the carrier at the same time the bolt is getting cam'd (read a hell of a lot of torque of the key to the side of the upper track since the spent case is now fire formed in the chamber, and the bolt has to fight against this force as well). Bottom line is to lube and shoot, then lube and shoot some more, and so on until the key mates (has it's way) with the upper reciever track.
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The problem is definitely getting worse, i.e. fewer and fewer rounds between cleanings/malfs. Using .223 headspace gages, and in comparison to other 5.56 chambers, this one appears a tad short (or small??). Is the dry film wearing off common? My Colts do not exhibit this condition. |
So far, it appears to be two of the listed items (headspace now listed as gauging tight on a 223 gauge), and God knowing, could be the third as well. Forget trying to break the upper in, its time to just send back the upper to be checked out/corrected by the supplier. |
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