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Posted: 1/7/2010 1:18:00 AM EDT
| I've been piecing together an AR for a while using a preban Essential Arms J-15 lower to remain compliant in NY and a DPMS upper. The rifle functions for a few rounds with good ejection before partially feeding a round and jamming. The round is pushed halfway out of the magazine before the bolt rides over the top of the case and crushes the top in, lodging the round with the bullet pointed toward the top of the barrel nut at a 45 degree angle. I suspected short stroking but think I have ruled it out. I have replaced the gas tube (Olympic Arms), bolt carrier group, buffer spring, and have sent the upper out for professionally cut M-4 ramps. Also used new NHTMG and Kay industries GI aluminum mags with mag pul self leveling followers. Ammo used has been Remington UMC, Lake City 55 grain FMJ and SS-109. Appreciate any input. Running out of ideas and need to make this a reliable weapon. Also looking for a decent company or gunsmith who would assist. Contacted DPMS. They didnt want to touch it because it was a DIY rifle. |
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First off, welcome to the site!!!!!
Now lets down to it, Rifle correctly cleaned (including the chamber via a chamber brush by hand) and the upper bearing areas CLP lubed, take a mag and load only a single round into it. Insert and charge the round, and while leaving the empty mag in the rifle, fire that round. If the rifle is stroking correctly, then the bolt will lock back on the bolt catch after the round has been fired. We first need to confirm if the rifle is full stroking, which could be the problem in it's self. Problem with this can be a simple as the cleaning and lubing agents being used, the gas tube not being index with the key correctly, gas leaks, hammer wedging, wrong recoils spring being used, and even a major gas leak or blockage. And trust me, if you have a jam with a the nose of the round in the chamber, and the bolt at rest on the middle/top of the case (not behind the case rim), then you either have short stroking, or a mag that is not recovering correctly. |
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Quoted:
I have had similar issues w/ FTF. Discovered that my rounds weren't seated horizontally in the magazine (nose canted upward) After my discovery, haven't had a single issue. Simple check. A quick check on such is to load three round, then shove the back, then the front of the top round in the mag down, then let the stack recover under it's own spring tension (also help to do the test afterwards with one more added round a well). The top round should recover with it's entire body tight against the mag feed lip. If it does not, pull the mag apart and give it a good CLP cleaning to see if that fixes the problem. |
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Thank you both for your input.
Rifle is usually cleaned maticulously. I even steal swabs from nurses to get the nooks and crannies. No dirt or debris in action at all. Thin coat of break free on the bolt and bolt carrier. Bolt locks to the rear when one round is fired on an empty mag. No gas leaks any where I can see. Checked around gas block/ barrel, gas tube, gas key. Nothing unusual. Gas key is indexing gas tube correctly. No dings or wear on tube or key. Locks up perfectly every time when functioned by hand. Gas key is staked properly. Solid. No leaks. Normal wear on hammer/ bolt. Does not appear to be hanging up. Smooth operation when bolt is operated by hand. Not feeling any thing rough or catching. Removed buffer and buffer spring. Tube is clean. One slight wear mark in tube. Checked buffer. It had some rough edges. Smoothed them out. Planning on replacing the buffer. After the above, the same issue exists, however the bolt is landing more towards the rear, not the middle of the case. 
Have not attempted the magazine check as of yet. Will do. I typically clean the mags whenever I clean the rifle, brushing out the feed lips, running a cloth through the magazine body, wiping the spring down with an oiled patch and wiping off the follower. Thank you again. |
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If the bolt is locking back, then the stoke is fine.
I would either suspect that either the mag is not being retained high enough in the mag well for a correct bolt strip, or as stated, the mag is not recovering fast enough to get the top round up high enough to be stripped correctly by the bolt. Since you did not state that there was both a spent round, and a live round in the chamber, it does not sound like a weak ejector problem. Also, since the bolt is coming to rest on top of the case, and not behind the rim during the jam, you can weed out the hammer catching the FP collar as well (tip of the round would have slightly entered the barrel extension, and the bolt locked up without even the FA being able to push it forward). If you need to do a quick check with the mag at hand to confirm that it has a weak spring and the problem, you can always pull the spring out of the mag and hand stretch it (cleaning the mag may help as well). Granted that the mag spring will return to it's old set tension pretty quickly, the tweak should last long enough to confirm if it's the mag or mag catch being the problem at hand. |
| Made another check on the range with fellow armorer/GEGN. He supplied me with a new lower for comparison. Lower was not set up for immeidate use, but noticed when magazines were inserted there was more of a gap in front of the Essential Arms magazine well. This allowed the magazines more play. We both fired the weapon to eliminate user error. Experienced same problems with full mags. No malfunctions on mags loaded with 10 rounds. My friend maintained a different grip on the rifle as well holding his off hand on the magazine well and putting rearward pressure on the magazine. I used feeler gauges to measure the gap in the magazine well. Essential Arms reciver had a gap of .032 in. The new DPMS had a gap of .017 in. with little play. My theory is spring pressure from a fully loaded magazine rocks the magazine forward placing the cartridge rim to far under the bolt to feed correctly. Any ideas on correction? Will be swaping out the lowers to confirm. |
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Granted that the mag should have some slop in the well so it can drop free, but if too much and the mag is swinging in the breeze, it will cause problems as well.
Start off by with the bolt closed, and with only a stripped mag body, hold the mag release button all the way in, and push the mag body in the well all the way up (past the catch), until the mag lips max out against the bottom of the carrier. Now release the mag catch button, and see how far down the mag will come until the mag catch device locks into play (including the amount until the catch device is tight against the top of the mag catch slot as well). About a 1/4" max play is all that you want. If you find more than this, then is may be time to start playing with/searching for a mag catch to find one that holds the mag up higher in the well so you only have the 1/4" max. On the same subject are you sure that the threaded part of the mag catch threaded portion is flush with the outer face of the button. If so, have you tried to thread the catch device in another thread on the button so it retains the mag with greater tension to solve the problem? |
| Swapped lower recievers. Same issue occuring. TM-220 manual states bolt catch may produce fail to feed issues. Believe that possiblity has been eliminated with the reciever switch. I have not tried measuring the magazine as suggested above will do ASAP. Noticed that my carbine has a heavy buffer. Was fortunate enough to be near multiple AR's and an electronic scale. Pulled buffers out of two weapons. Mine was almost a full ounce heavier than the standard buffer. Slowing the bolt down to much when cycling forward? |
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You stated that the bolt will lock back on the last shot, so the weight of the buffer is fine (no causing short stroking), and with the amount of reward stall due to such, should not be the problem in the match.
But will add, are you sure that the bolt catch is locking the bolt back via the bolt face, and not just the catch catching only the bottom of the carrier alone? |
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Catch is locking on the bolt face.
Believe the issue is magazine intermitently rocking downward causing the tip of the round to catch on the end of the ramp and skip up bringing the case rim under the bolt as it is traveling forward. Not into altering recievers. Beilieve my only option at this point is to try to bef up a magazine to decrease play in the mag well and get rid of the slop. Hopefully forcing teh round to remain aligned with the feed ramp I will eliminate all of the failures. Ordered some polymer P-mags. Going to test them first for fit, if the same issue, planning on attching another piece of polymer to the face of the magazine to eliminate play. Thoughts? |
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