billdozer Posted June 17, 2014 Share Posted June 17, 2014 This is a new build. The mag works fine in my other ar's. Will just tightening the magazine catch help? Or am I looking at modifying or trying different catches to get the mag to seat higher? Link to comment Share on other sites More sharing options...
alma Posted June 17, 2014 Share Posted June 17, 2014 Any chance this is with a new Pmag loaded all the way up? Link to comment Share on other sites More sharing options...
billdozer Posted June 17, 2014 Author Share Posted June 17, 2014 No, it's a 20 round pmag. Link to comment Share on other sites More sharing options...
a.roberts Posted June 17, 2014 Share Posted June 17, 2014 Does the bcg lock back on an empty mag? If it doesn't, it might be short-stroking. Link to comment Share on other sites More sharing options...
billdozer Posted June 17, 2014 Author Share Posted June 17, 2014 It does lock back. Link to comment Share on other sites More sharing options...
BitchinCamaro Posted June 17, 2014 Share Posted June 17, 2014 Does the bolt face make contact with the cartridge case at all? Link to comment Share on other sites More sharing options...
billdozer Posted June 17, 2014 Author Share Posted June 17, 2014 It seems to do it more with the bullet on the dust cover side of the stack. Link to comment Share on other sites More sharing options...
Shooter115 Posted June 17, 2014 Share Posted June 17, 2014 Does it cycle fine manually? Link to comment Share on other sites More sharing options...
Scott Steele Posted June 17, 2014 Share Posted June 17, 2014 It's possible your BCG is under or over gassed. Do you have an adjustable gas block? Link to comment Share on other sites More sharing options...
Scott Steele Posted June 17, 2014 Share Posted June 17, 2014 Put one round in your mag and fire it to see if the BCG will lock back. If not, and you have an adjustable gas block, turn the gas off and work your way up till it locks back and then give it a little more gas for reliability. If it is over gassed the BCG moves right over the next round because the mag spring isn't fast enough to push the next round up fast enough. hope this helps. Link to comment Share on other sites More sharing options...
billdozer Posted June 17, 2014 Author Share Posted June 17, 2014 (edited) It's an adjustable gas block, it's adjusted to lock back, just barely Edited June 17, 2014 by billdozer Link to comment Share on other sites More sharing options...
a.roberts Posted June 17, 2014 Share Posted June 17, 2014 Is the 20rd pmag the only mag this occurs with? Link to comment Share on other sites More sharing options...
Stlhead Posted June 17, 2014 Share Posted June 17, 2014 Just to rule out under gassed, open up the gas another 1/2 turn, then load 1 into a mag and shoot it to check for lock back 10 times, then load 2 and repeat 10 times, then load three. If it locks back at every empty then you have plenty of gas. If you fail to feed or fail to lock back intermittently I would recheck the gas system as well as the ammo. Also check for other drags on the bolt carrier or buffer that may be affecting the cycle. Buffer pin? Gas tube not aligned and dragging, bent buffer tube, loose carrier key? Lots of things can cause a new gun not to run right, just start eliminating things one at a time until it runs. One thing I can say with a good deal of certainty is that the mag catch is not the problem. Link to comment Share on other sites More sharing options...
BitchinCamaro Posted June 17, 2014 Share Posted June 17, 2014 Second that. Though I don't see immediately how system that locks back on an empty mag wouldn't also strip a round from the mag, open up the gas and rule it out first. Also, after a few months with my adjustable block I'm finding that I like to have a 1/2 to a full turn open from my minimum setting. There are too many variables in ammo to keep within the lower margins of error. What gas length are you using? Link to comment Share on other sites More sharing options...
billdozer Posted June 18, 2014 Author Share Posted June 18, 2014 Rifle gas. Fairly confident it's not under/over gassed. I don't think it's the mag catch, it was just a thought. I haven't got another chance to go back out with different mags. I usually use this mag because is easier to bag the rifle with it. I'll update with more info tomorrow. Link to comment Share on other sites More sharing options...
Mark Gale Posted June 18, 2014 Share Posted June 18, 2014 (edited) If it is over gassed the BCG moves right over the next round because the mag spring isn't fast enough to push the next round up fast enough. Nonsense... First question, please explain how a gas charge that operates the recoil phase of the cycle of operation has any effect at all on the speed of the bolt group during the counter recoil phase. Second question, given that top cartridge in the ammunition stack is resting against the bottom of the bolt group when the chambered round is fired, what action creates the force to displace the ammunition stack downward far enough to allow the bolt group to skip over said top cartridge. Edited June 18, 2014 by Mark Gale Link to comment Share on other sites More sharing options...
Scott Steele Posted June 18, 2014 Share Posted June 18, 2014 If it is over gassed the BCG moves right over the next round because the mag spring isn't fast enough to push the next round up fast enough. Nonsense... First question, please explain how a gas charge that operates the recoil phase of the cycle of operation has any effect at all on the speed of the bolt group during the counter recoil phase. Second question, given that top cartridge in the ammunition stack is resting against the bottom of the bolt group when the chambered round is fired, what action creates the force to displace the ammunition stack downward far enough to allow the bolt group to skip over said top cartridge. oooops, had bolt override thought wrong. My bad. Billdozer, definitely try another magazine to see if it still happens. Link to comment Share on other sites More sharing options...
Shooter116 Posted June 18, 2014 Share Posted June 18, 2014 Are you running a Low Mass Buffer? If so pull it out and check the roll pin in the end near the rubber end plate. These work out occasionally and will drag on the buffer tube slowing down down the BCG. I have had this happen several times as have others I know. Take a punch and widen up the ends of the roll pin to keep it in place. Link to comment Share on other sites More sharing options...
Doug H. Posted June 18, 2014 Share Posted June 18, 2014 If it is over gassed the BCG moves right over the next round because the mag spring isn't fast enough to push the next round up fast enough. Nonsense... First question, please explain how a gas charge that operates the recoil phase of the cycle of operation has any effect at all on the speed of the bolt group during the counter recoil phase. Second question, given that top cartridge in the ammunition stack is resting against the bottom of the bolt group when the chambered round is fired, what action creates the force to displace the ammunition stack downward far enough to allow the bolt group to skip over said top cartridge. This may sound like nonsense but I had this exact situation happen to me this year at Nationals. Shooting .223 in my JP with low mass carrier, red buffer and 1 year old recoil spring and the gun runs perfect. With 5.56 (Winchester Q3131) it will often fail to pick up the next round. Go back to .223 works perfect. Tried 4 different mags and get the same result. Hard for me to believe that the 5.56 is not causing the problem. None of my other rifles does this, just the JP tuned for light loads. Doug Link to comment Share on other sites More sharing options...
billdozer Posted June 19, 2014 Author Share Posted June 19, 2014 Tried a different magazines today Better, but still not 100%. I'm gonna try swapping the rca low mass for my jp. Then swap my Taccom ulw buffer for my jp captured spring. Link to comment Share on other sites More sharing options...
openclassterror Posted June 19, 2014 Share Posted June 19, 2014 (edited) If it is over gassed the BCG moves right over the next round because the mag spring isn't fast enough to push the next round up fast enough. Nonsense... First question, please explain how a gas charge that operates the recoil phase of the cycle of operation has any effect at all on the speed of the bolt group during the counter recoil phase. Here is how. If the rifle is under gassed, or properly gassed, the bolt has just enough energy to fully retract, and no more. All energy is expended by the end of the stroke, and there is a slight dwell as the bolt slows at full spring compression and reverses direction. Ideally, it should fully expend its cycling energy WITHOUT bottoming out the buffer in the buffer tube, or just barely bump at most. If overgassed, the rearward bolt cycle velocity is higher, and the buffer bottoms out in the buffer tube at full speed. Spring is at this point fully compressed, and bolt is fully stopped from rearward movement, but the time to get to the point of reversal is significantly reduced. While not exactly affecting the speed of the return stroke, it IS affecting the time elapsed before the bolt passes back over the magazine, as the recoil stroke is measurably faster when over-gassed. If the magazine is at all sluggish, the next round does not have time to pop up prior to the bolt face passing the rear of the cartridge. USUALLY (not always) this malfunction has a round partway out of the magazine with a huge crease in the side from the bolt lugs trying to strip it forward from above instead of behind. Typically if undergassed, the bolt retracts just far enough for the empty to clear the ejection port, and bolt closes on an empty chamber without stripping a round from the mag. If it intermittently mixes full stroke (bolt lock on empty mag) with symptoms of short stroke (empty chamber) it often means mechanical interference. The new Gen of PMags have slightly reshaped feed lips, and sometimes drag on aftermarket carriers with non-milspec dimensioned grooves on the bottom. Problems can be intermittent, especially if gas tube is not centered. This can cause the carrier to rotate VERY slightly in the receiver bore from shot to shot, and drag on one feed lip or the other, and sometimes neither. My JP LMOS carrier runs fine with old PMags and Brownell's Aluminum mags, but has had rubbing issues with the newest Magpuls. Just a thought (or 5 ) Edited June 19, 2014 by openclassterror Link to comment Share on other sites More sharing options...
Mark Gale Posted June 19, 2014 Share Posted June 19, 2014 (edited) @Doug H. - I can't argue with you're experience, but you may be confusing short stroking with a bolt over base malfunction. Lets face it, Winchester Q3131 isn't known to be a high quality ammunition. Unless you have some chrono data, the 223 (whose) may have been supplying more gas than you think. What in the name of Baby Jesus were you doing with Crapchester ammunition at Nationals? @openclassterror - There is so much wrong with your post I'm not sure where to start, so I won't. Given a properly built rifle, except for a gas port that is excessively large, and a serviceable magazine, the bolt group can't out run the ammunition stack, period, full stop. 99% of the time, intermittent malfunctions are the result of poor quality parts (read out of spec), junk ammunition, inadequate lubrication or some combination thereof. Our first clue in the OP's case is his opening statement, "this is a new build". His failure to give specifics as to the make of the parts comprising his new amalgamation is the second clue. He'll never be able to solve his problem until, through methodical diagnosis, he uncovers what the problem is... Edited June 19, 2014 by Mark Gale Link to comment Share on other sites More sharing options...
Doug H. Posted June 19, 2014 Share Posted June 19, 2014 @Doug H. - I can't argue with you're experience, but you may be confusing short stroking with a bolt over base malfunction. Lets face it, Winchester Q3131 isn't known to be a high quality ammunition. Unless you have some chrono data, the 223 (whose) may have been supplying more gas than you think. What in the name of Baby Jesus were you doing with Crapchester ammunition at Nationals? @openclassterror - There is so much wrong with your post I'm not sure where to start, so I won't. Given a properly built rifle, except for a gas port that is excessively large, and a serviceable magazine, the bolt group can't out run the ammunition stack, period, full stop. 99% of the time, intermittent malfunctions are the result of poor quality parts (read out of spec), junk ammunition, inadequate lubrication or some combination thereof. Our first clue in the OP's case is his opening statement, "this is a new build". His failure to give specifics as to the make of the parts comprising his new amalgamation is the second clue. He'll never be able to solve his problem until, through methodical diagnosis, he uncovers what the problem is... Win Q3131 not high quality? Compared to what? Do you have evidence of this or is this your person opinion? For hoser ammo I shoot what I can get cheap and runs in my gun. Problem was I had not checked the Q3131 in my competition rifle except to zero and it was all I had available at home. All though I'm probably not an AR expert, I've sent a few rounds down range and also have had similar experience with AK type weapons. Excessive bolt speed can lead to malfunctions. When you lighten the bolt carrier and tune the gas system you reduce the operational range of the gun. Use ammo that is not in that window and you will have problems. Without high speed video of the exact malfunction neither of us can say exactly what is happening but be careful with making pronouncements about what "can't" happen. Doug Link to comment Share on other sites More sharing options...
openclassterror Posted June 19, 2014 Share Posted June 19, 2014 @openclassterror - There is so much wrong with your post I'm not sure where to start, so I won't. OK,I guess using high speed video for troubleshooting and spending 12 years working professionally on ARs doesn't qualify me to speak. We have been building and experimenting with adjustable gas systems on ARs for a number of years, and there is no question that increased velocity on the opening movement reduces the time the bolt spends in reciprocation. We use video to measure everything from bolt dwell time(receiver side wall is cut open so we can record lapse between hammer fall and bolt movement) to bolt bounce on closure. Notice I did not say that this is always the cause of this particular malfunction. Due to our design work we have a great deal of film on ARs over a wide range of gas input, and I am only describing situations we have seen FIRST HAND and can verify through replay. You are welcome to have any of your own opinions that you want, but you don't know me or what I do for a living so don't impugn my input if you don't know how it was obtained. Bolt-over-base malfs can have several causes. That is one reason that remote diagnosis is difficult. Almost ANY malfunction could have more than one cause. That is why I use words like "typically" or "often". I also don't usually venture to suggest the most common causes because a lot of guys can offer that info. I am simply adding what I can say from verified first hand experience to the conversation in case the usual suspects are not at fault. Link to comment Share on other sites More sharing options...
Stlhead Posted June 19, 2014 Share Posted June 19, 2014 To all involved in the pissing match over what is and is not possible, your debate is not helping the OP. To the OP, If you change out the buffer and or the bolt carrier or spring before eliminating other possibilities you are just throwing in more variables to the equation. Even if you are able to get it to work you will have done so without a direct knowledge of exactly what you did. Randomly changing parts for different ones is poor troubleshooting and repair methodology. You could change out parts 1 at a time for exact replacements if you suspect an individual part is to blame, but changing to totally different parts will certainly introduce more problems in the long run unless it is your goal to use the new parts exclusively from now on, if that is the case then why did you start with the parts in your build to begin with? If you want more help from the peanut gallery, you should give us more information, and you may want to be more open to the info you get. If you do not think that there is any merit to the info you have been given then why did you waste everyone's time asking in the first place if you where already so much smarter than the folks that have tried to help you. I understand how frustrating problems like this can be, but steady and methodical troubleshooting will get to the bottom of it if you stick with it. If you are not willing or able to then perhaps you should consider taking the gun to a reputable local smith or possibly selling it to a buddy who is willing to take on the troubleshooting challenge. Then buy a JP or other turn key solution that offers support. Link to comment Share on other sites More sharing options...
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