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Posted: 10/25/2002 1:19:44 PM EDT
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Hello to all, Need help with RR CMP type rifle short stroking. Here are the facts. Matched RR upper and lower, Merchant post ban SS barrell A2 buttstock Steel float tube Bolt and Carrier Assembly from Brownells RR 2 stage trigger Firing BHM 68gr or BHM 75gr NEW Brass looks good no machine marks....post firing Well lubed and clean. Extremely smooth during manual charging, slight amount of resistance as Carrier goes over bolt catch with clip in. Using several of the many colt clips I have with single round follower, rifle fires. Carrier stops on clip latch, instead of bolt face stopping there. Will eject spent rounds. 3 out of 4 anyway. Loading up 3-4 or however many rounds in regular clip, first round fires, maybe gets ejected, fails to strip off next round, or strips off next round and trys to feed it past spent round that does not get ejected. Have done this so far. Re-cleaned and lubed x 5 Polished chamber Inspected and triple checked gas system for alignment and blockages Changed buffer spring Polished tip of hammer Polished bottom side of Carrier where hammer and clip latch touches. Gas rings good I get a small amount of carbon residue on top of barrel in front of where front sight gas port and barrel gas port line up. Question. What is the minimum gas port diameter on the barrel? This is really blowing my mind! I really need your help thanks to all |
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Lets start with ammo, then the rifle. The thing with hand loads is that they need to be checked for size and pressure. If you’re using the wrong dies, or set up incorrectly, you’re going to have problems. As for your reloading powder specs, you may be loading too hot, and your short stroking may be signs of high pressure. Post what your spec's are and maybe we can see if you’re over the top. The rifle. The one thing that you may want to check is the chamber. Since you stated that you have purchased the barrel, then the bolt separately, you might be on the tight side. On my match barrels, I send the bolt in to have the chamber matched/cut to the bolt. On yours, it sounds like you just purchased the bolt and barrel and did not match the two. On the barrel port with the use of the adjustable tower, it’s possible to offset the tower to the gas port. If the barrel was installed without indexing the port to the upper, you could be running into problems when you zero the sight to upper. Compass lake has a solution to resolve the off set by beveling the top of the port to allow for a slight offset. www.compasslake.com/float_tube.htm The rifle is new and tight, including the bolt to barrel extension. If the barrel was not installed true to indexed, the bolt could be binding up on exit. Try to work the bolt by hand and see if the carrier lugs and a bolt lugs are aligned. The bolt should enter the extension without touching the side’s f the barrel extension. If it is touching on entry, add this to a tight throat area/chamber and you get short stroke city. As for carrier and bolt, you may want to check the key for leaks. On some, the key needs to be lapped to the carrier to prevent leaks. Also check the key passage to carrier passage. Some have known to be out of alignment and need the part brought back into alignment. If you find that the upper does check out, then it time to look at the lower. Since you didn't state what trigger your using, I would check the disconnector for binding. Binding will leave a wear line just behind the disconnector hook. Next on to the buffer and spring, check them for binding. Pull the buffer and check the roll pin. See if the pin is extending out and binding up on the spring. If so, use a file and contour the roll pin to match the sides of the buffer. Then check the spring. It should be tapered to retain the buffer lip. Then check the butt stock screw for length. It may be too long and is extending into the receiver extension (tube). If you forgot to install the a-2 extension cap, this will definitely allow the screw to enter the tube and cause the buffer to bottom out against the screw, verses the end of the tube. Truthfully, It sounds like it time to try some factory loads to see it the rifle is at fault or the ammo. If it's the ammo, time to start asking for a load that is proven. If it's the rifle, then it's time for a strip down and a slower re-build to check all the parts out for spec and alignment. P.S. Welcome to Ar-15 and Gunsmithing 101. |
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Dano had it covered. Try factory loads...even some Wolf ammo. There's always someone at the range with a bag full. The chamber has to be suspect only because a) it's new & unproven, and b) You did not headspace to make sure the bolt and chanber are happy together. THis is fairly important...especially where custom barrels are concerned. THings may be just a little tight, and like Dano says, you could be approaching pressure problems...in more ways than one. Have you used these loads in other guns? You ay you put in a 2 stage trigger, but you didn't say where the other Fire Control Group parts came from. Is the hammer of the long-tail variety? Are you sure the springs are all in correctly? Did you mount the barrel? If so, did you use vise blocks for the upper? I would stay away from any more polshing. It sounds like you polished & waxed everything but your neighbors' car. Polishing is a nice pass-time (I polished the heck out of my AK-47 parts one day) but AR15 fire-control parts are usually only surface hardened and you may be shortening their usable life. |
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To all, Thanks for the initial help, I do appreciate it. Just some clarifications from my initial post. BMH stands for(Black Hills Match) ammunition, it is new, not reloaded. Sorry should have been more specific. Does the same thing on Federal Gold Medal Match, New ammunition as well. Barrel and bolt were purchased seperately and have been head spaced/indexed using all appropriate tools. As you say the Fire Control Group, it has been specified as a RR 2 stage, pretty much stands for Rock River Two Stage Trigger. Should have been more specific on that as well, sorry, I'm new to the board. Buffer spring without binding. No signs of wear behind disconnector hook. A2 Extension cap intact. Merchant is well known for their tight chambers, and as all of you have stated, I too am suspecting that the chamber may be to tight! However the spent casings do not exhibit any signs of such? This is certainly a tough one. Any other ideas? Many thanks to you all! |
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susrn, you just answered your own question. Your shooting ammo that has not been check in rifle. You can get over pressure not only by the rounds being loaded too hot, but by the following: 1. Tight chamber. 2. Tight head space. 3. Tight barrel throat. Since you say that the rest of the rifle checks out, you want to take bets on how may of theses are the problem on your rifle? I am going to go with at leasts two out of the three. The simple solution is to send the barrel and bolt off to be re-chambered and throated for the ammo that you are shooting. It should not be too much as long as the smith does not have to set the barrel back. Hope this helps. |
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Gas port diameter for rifles (standard 20" barrel) is 0.091". Derrick Martin (Accuracy Speaks) admits in his book to opening his rifles to 0.099" for reliability in locking open from standing. I would treat that as a last resort move only. Make sure that the front sight tower is indeed lined up on your gas port. When you remove the gas port, there will be carbon and ash outlining where the edge of the hole was... Dano mentioned carrier key fit, so I will just say that this has happened. The fact that the empties are not showing signs of being hot makes me wonder. Lots of folks shoot this stuff in competition. Are you cetain that there is no cratering, no piercing, no leaking, no little plugs of case shoved back into the ejector hole, and no embossed marks from the chamber on the cases? On the topic of small chambers, before you commit to a rechambering, look at those fired cases closely. Seperate the upper and lower and put the bolt and carrier out of reach. New match cases should fall into the chamber with a click, and fall back out too. So should match ammo. If it sticks, use a rod to get it out - please, do not tempt fate by assembling the gun. We keep seeing symptoms of tight chambers on this forum. Even for competition, a tight chamber is a liability not an asset. One of the best competition chambers around is the Wylde. The Army wins just about everything they enter using a slightly modified Wylde. Others use a shorter throat than the Wylde, but they work. These competition chambers may not love foriegn military surplus ammo, but they should eat Black Hills and Federal Gold Medal just fine. Derrick Martin (Accuracy Speaks) and Frank White (Compass Lake Engineering) and John Holliger (White Oak) all build fantastic rifles with chambers that just work. I had Mo Defina (MCS in Connecticut) chamber my current barrel, and it just works. If you think that the chamber is small and have backed it up with tests, get in touch with one of these fine folks and you ought to be OK. |
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