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Posted: 11/10/2016 6:59:12 PM EDT
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OK, I've got a "lightweight" build I did and I'm having a hell of a time getting it to feed correctly.
It happens fairly regularly, so I don't think it's anything super obscure. Thing's I've tried: Lightweight Taccom buffer, also PSA standard carbine buffer 10% reduced spring, Tubbs spring, standard carbine spring Lightweight Faxxon BCG.. I also just tried a PSA premium BCG today, same issue Multiple magazines Various gas settings.. I have the SLR gas block, seems to work fine and I adjust it to LRBHO and it works fine. I've tried giving it one more click too. Any ideas? My cartridge is all dented up when this happens from the bolt closing on it. I'm using the RA-140 trigger, possible something with the hammer on that? Barrel/feed ramp issue? Images: 
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I had a similar issue with a similar setup. Have you tried polishing your feed ramps and using a stronger spring? What barrel? Does your brass get dented?
The Faxon barrel extension is sharp and needs to be polished imo. That's what fixed it for me. I used port much every part you listed but used a v7 ti carrier |
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It's happened with different ammo, today was American Eagle 223..
I don't think it's too much grease, it happened on previous BCG too I don't have a ton on it's just white so stands out and that picture was a few shots into the brand new bolt. It seems like the bolt is missing the rim of the cartridge somehow or something. |
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Random short stroking = wrong lube (grease is bad mmmkay), gas leak, undergassed, or binding. or it is cycling way too gas and your mags are defective.
I'd clean and lube that with clp only, use a known good USGI mag, set the gas right, standard spring, and go to town. I'd also check for a gas leak at the carrier key, and good gas tube alignment at the key. |
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Quoted:
OK, I've got a "lightweight" build I did and I'm having a hell of a time getting it to feed correctly. It happens fairly regularly, so I don't think it's anything super obscure. Any ideas? get rid of all the dumb stuff, get a standard BCG/spring buffer & gas wide open lube with clp=see if it runs (its a common mistake on newbies to put "highspeed" stuff in their gun to make it not run) If that fixes it, then replace one part at a time and adjust the gas accordingly if it does not fix it, you have a gas problem or possibel feeding problem load 1 round and see if it holds the BC back on a empty mag, if not=gas problem eta if you are adamant about running the lighweight stuff, then break the edges on the lugs of the barrel extension, it will reduce the force needed to chamber a round (realize this is not a fix, its a bandaid to run non standard parts |
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Looks to me like a bolt over base, which generally means your either cycling way to fast....or your not cycling far enough back. This would be my guess as well. If you were motivated to experiment... I'd try sanding some length off of the back end of the buffer. Or a stronger spring... to help with any possible buffer bounce. Also try some very weak ammo, like Wolf, to see if that works. |
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That is our diagnosis as well. Quoted:
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Looks to me like a bolt over base, which generally means your either cycling way to fast....or your not cycling far enough back. That is our diagnosis as well. So what direction is the brass being thrown? Forward, back, or eradic? Also, can't see pics at work. Barrel and gas system lengths please? |
| When you adjusted the block did you shoot several mags to LRBHO or only one? To me it looks like Its short stroking and its probably on the edge of a short stroke and that is why it happens at random with multiple mags. You need more gas. Open up the SLR block to it max setting for a baseline in testing and if its still is doing it then I would check the gas blocks alignment. |
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When you adjusted the block did you shoot several mags to LRBHO or only one? To me it looks like Its short stroking and its probably on the edge of a short stroke and that is why it happens at random with multiple mags. You need more gas. Open up the SLR block to it max setting for a baseline in testing and if its still is doing it then I would check the gas blocks alignment. Won't really know until we hear what direction the brass is going and what the recoil feels like. Sharp and harsh means it's cycling too fast, and the mag can't feed rounds fast enough. Also the brass would be ejecting to either the front or all over the place. |
| Does your spent brass have any aggressive ejection marks at the base to indicate overgassed?...aggressive swipes and deep, hard digs at the rim? When reloading ammo, this always tells me a good deal about gas levels and ejection. Try a different mag or two for an easy check just to eliminate that. |
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Does your spent brass have any aggressive ejection marks at the base to indicate overgassed?...aggressive swipes and deep, hard digs at the rim? When reloading ammo, this always tells me a good deal about gas levels and ejection. Try a different mag or two for an easy check just to eliminate that. This is very useful advice. Actual ejection location (hands of the clock) is far less useful and can be very misleading. |
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When you adjusted the block did you shoot several mags to LRBHO or only one? To me it looks like Its short stroking and its probably on the edge of a short stroke and that is why it happens at random with multiple mags. You need more gas. Open up the SLR block to it max setting for a baseline in testing and if its still is doing it then I would check the gas blocks alignment. This I had a gas block move very slightly and it was enough to make it short cycle. |
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First, much appreciate all the feedback and suggestions.
So as of now I am not running anything oddball, most recent set of failures were similar to my "lightweight" build failures. Right now I have: PSA Premium BCG, PSA carbine buffer, and a Tubbs spring. I did try a standard carbine spring as well. I will try stripping the grease off and just using oil (EWL or CLP), but it's not caked on grease or anything and there's oil too. The suggestion to mix-match uppers/lowers was good too, I'll try that - I have a 6920 I will use for that. I'm pretty confident in my gas setting. I shot like 6-8 rounds in a row with a single round and it locked the bold back each time. One click back and no lock back, and I tried an additional click as well. I may go "wide open" since I have full weight buffer/bolt in now and see if that does it. Ejection pattern looked good - it was generally 2-3 oclock. So I think I'll try the above suggestions and I'll also try carbine spring again, hopefully I can narrow it down more. |
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Did you say what ammo you are using? Remember if you switch ammo you could possibly need more gas.
One click above locking back every time MIGHT not be sufficient, on an adjustable gas block. For troubleshooting, I ALWAYS open it fully. No reason not too. There is more drag on a carrier with a full magazine, than an empty one. Additionally, I did have a friend's build with a custom higher end trigger - cause short stroking randomly. His hammer was dragging on the bottom of the carrier with way too much force.... I ended up filing the top of his hammer to reduce this and it didnt take much, but made a huge difference. I also have a rifle where the disconnector was getting POUNDED because the hammer was interfering just a bit too much with the carrier - same solution. So don't rule out swapping the lower or trigger. Ditch that tubbs spring. Lastly - another random issue I had was with PMAGS.... gen 3 pmags on one of my carriers wont work together. The lips on the mags rub on the sides of the carrier, and slow it down with my otherwise perfect adjustable gas system.... causing it to slow down and short stroke. |
If you are having your brass eject at 2-3 o clock you are more than likely not overgassed. You need more gas. I think you are just barely short stroking with the amount of gas you have going through the SLR adjustable block. Report back once you have opened the gas block to its highest setting and run a few mags through her. Looking forward to your post. Use a few GI mags or lancer mags if you got them. Oil the crap out of the BCG too 
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Random short stroking = wrong lube (grease is bad mmmkay), gas leak, undergassed, or binding. or it is cycling way too gas and your mags are defective. I'd clean and lube that with clp only, use a known good USGI mag, set the gas right, standard spring, and go to town. I'd also check for a gas leak at the carrier key, and good gas tube alignment at the key. Grease is okay. I use tw25b grease myself and found it to be the best thing I ever used. However, you need a very small amount. The OP's pictures look a bit heavy if you're able to see the mixing compound. I think he would need to use less and spread it out more so that it is so thin you can barely see it. |
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Grease is okay. I use tw25b grease myself and found it to be the best thing I ever used. However, you need a very small amount. The OP's pictures look a bit heavy if you're able to see the mixing compound. I think he would need to use less and spread it out more so that it is so thin you can barely see it. Quoted:
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Random short stroking = wrong lube (grease is bad mmmkay), gas leak, undergassed, or binding. or it is cycling way too gas and your mags are defective. I'd clean and lube that with clp only, use a known good USGI mag, set the gas right, standard spring, and go to town. I'd also check for a gas leak at the carrier key, and good gas tube alignment at the key. Grease is okay. I use tw25b grease myself and found it to be the best thing I ever used. However, you need a very small amount. The OP's pictures look a bit heavy if you're able to see the mixing compound. I think he would need to use less and spread it out more so that it is so thin you can barely see it. I have seen more people short stroking at the range and in competition - because of using grease. It seems fine at first, but once it gets hot, then cold, and lots of carbon picked up in there - the bolt really starts slowing down. ESPECIALLY when using an adjustable gas system, where there isnt enough overgassing to overcome this added friction. It may be "ok" for people who do a good job of keeping things clean, and use a grease that stays viscous enough in the cold, and don't shoot too much between cleanings to get extreme carbon buildup..... but for high volume, I am not a fan, at least on AR15 carrier bearing surfaces. |
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OK - finally got a chance to go out and mix-match with my Colt. It's the upper for sure, lower worked fine with the Colt upper, same FTF when I used my upper with Colt lower.
I've pretty much eliminated it being the bolt (since I've tried both my lightweight and normal bolts) and I did give it a wipe down and lube it up with oil beforehand. Which leaves me kind of dreading it a bit because it pretty much has to be my barrel install, no? I did have to fuss with the spacers on my ALG rail a bit more than I thought I should have to, maybe I've got a very small spacing issue with the barrel nut tightness/spacing? |
| One time this happened to me when I was using a crappy mag. I'm assuming you're using good mags. I didn't see if you said what you were using. FALARAK talked about using good USGI mags too and I didn't hear a response. It's probably not that simple, but I figured I'd throw it out there. |
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OK - finally got a chance to go out and mix-match with my Colt. It's the upper for sure, lower worked fine with the Colt upper, same FTF when I used my upper with Colt lower. I've pretty much eliminated it being the bolt (since I've tried both my lightweight and normal bolts) and I did give it a wipe down and lube it up with oil beforehand. Which leaves me kind of dreading it a bit because it pretty much has to be my barrel install, no? I did have to fuss with the spacers on my ALG rail a bit more than I thought I should have to, maybe I've got a very small spacing issue with the barrel nut tightness/spacing? I did try a few mags and the one I used that had FTFs in my upper worked fine with colt upper. That said, I'm desperate to avoid the hassle of pulling my barrel off and all that rigmarole so maybe I'll give it another shot with completely different mags next time. |
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OK - finally got a chance to go out and mix-match with my Colt. It's the upper for sure, lower worked fine with the Colt upper, same FTF when I used my upper with Colt lower. I've pretty much eliminated it being the bolt (since I've tried both my lightweight and normal bolts) and I did give it a wipe down and lube it up with oil beforehand. Which leaves me kind of dreading it a bit because it pretty much has to be my barrel install, no? I did have to fuss with the spacers on my ALG rail a bit more than I thought I should have to, maybe I've got a very small spacing issue with the barrel nut tightness/spacing? As long as the barrel nut is torqued, there should be no spacing issues unless something is out of spec. The spacers are just spacing the barrel nut itself from the extention, and if it's torqued properly the barrel will be seated into the upper. The best thing I think you should do is open the gas block COMPLETELY, keep the normal full auto bolt carrier, usgi buffer spring, usgi mag, and see what happens. I'd even go so far as just putting a normal, good quality low profile gas block on it and see what happens. |
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OK, I've tried with fully open gas block and tried a few different magazines. Same FTF issue.
So at this point I've ruled out (mostly, barring e.g. 2 bad bolts that have same FTF issue): lower bolt magazines What's left? Is it possible I need to polish my feed ramps? I can't get a good picture, but the ramp and upper have a small "ledge" I can feel with my finger. Nothing big or obvious, but it's the only thing left I can think of before I say F it and ship it off to a gunsmith. |
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Have you actually tried removing all the grease and lubing it "excessively" like Pat Rogers recommends with a gun oil? I'd makes sure to clean and oil the buffer spring too since there's a shit load of movement and potential friction there.
Also look at Hellbender's guide to tweaking an AR in the Precision Rifles section. |
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OK, I've tried with fully open gas block and tried a few different magazines. Same FTF issue. So at this point I've ruled out (mostly, barring e.g. 2 bad bolts that have same FTF issue): lower bolt magazines What's left? Is it possible I need to polish my feed ramps? I can't get a good picture, but the ramp and upper have a small "ledge" I can feel with my finger. Nothing big or obvious, but it's the only thing left I can think of before I say F it and ship it off to a gunsmith. Are you sure the gas block is properly aligned with the gas port? |
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I've used 3-4 different magazines, two types - Pmag and Brownell's GI style. These same magazines worked fine with my lower+Colt upper, so if it's magazine then it's still a problem as it's way too picky.
I did wipe off most of the grease and oil the shit out of it. And I have put some rounds through it by single-loading the magazine and shooting one at a time (don't want to waste time at the range I paid for). It shoots perfectly well when single-loading rounds. Magazine locks back every time even with 3-4 turns on the gas block, and I tried shooting with gas block wide open too so I doubt it's a gas problem though maybe. My current plan is to polish the ramp and smooth the transition a bit, then if that fails take barrel off, take gas block off, re-install carefully and try again. Then throw hands in air and give it to a gunsmith to fix. Any other ideas? |
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I have seen more people short stroking at the range and in competition - because of using grease. It seems fine at first, but once it gets hot, then cold, and lots of carbon picked up in there - the bolt really starts slowing down. ESPECIALLY when using an adjustable gas system, where there isnt enough overgassing to overcome this added friction. It may be "ok" for people who do a good job of keeping things clean, and use a grease that stays viscous enough in the cold, and don't shoot too much between cleanings to get extreme carbon buildup..... but for high volume, I am not a fan, at least on AR15 carrier bearing surfaces. That's wholly dependent on the grease. TW25B is NLGI Grade 1, which means... drumroll... it has a high viscosity index, which means, drum roll, the viscosity doesn't change significantly with temperature. Now, personally, I've used it from 0's to 100's, same shit. Brian Inos slide glide on the other hand, that'll lock my 1911s/2011s up like a brick below 40*. I almost exclusively use TW25B on my ARs for contact points, no problems. |
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I always like when a thread is ended with the resolution (for posterity) so...
I got it working (I think). Shot through 2 magazines with no FTF today. I did make a half-assed attempt to polish the feed ramps but it was too messy for me and hard to get the right angle with my big felt polisher/dremel so I gave up. Don't think I did much beyond scratch the paint. I took the whole thing down and remounted barrel and gas block, so could have been either thing. I very carefully aligned everything this time, not that I just slapped shit on haphazardly last time (or so I thought). I also added another shim to the ALG rail. I have a theory that last time the barrel nut may have stopped up against the ejection port door rod and I may have just had some sort of either sideways shim (very, very slight) to the nut, or something else off. Anyway, with attention to detail now it's working. Now to add back the lightweight shit and break it again probably 
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