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Posted: 1/2/2017 7:00:57 PM EDT
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Ok, finishing up my 8yr old's deer rifle for next year, an A3 build with SOPMOD 6-pos stock and 18" Ballistic Advantage barrel. I'm using a generic 0.750" low profile gas block, standard length gas tube, and free float keymod receiver. The bolt carrier group is a PSA $79 special, and the problem I am having is short stroking. The carrier will not travel far enough back to lock on an empty magazine, and I have tried multiple mags and Winchester Q3131 ammo. I've tried the BCG through another rifle, and it locks back fine...I've also tried the lower on another rifle to make sure its functioning 100%, and no problems there. I've also tried the problem upper on a functioning lower, still short stroking.
I've tried adjusting the gas block position, still the same problem. I've verified the gas tube is aligned inside the gas block, and attempted to measure the gas port in the barrel...mics at around 0.094". The only thing I can think is improper gas block alignment, but I set up a jig to run compressed air through the barrel, and have positioned the block when the air was flowing the loudest. Gas block port is definitely large enough, so I have no idea how to proceed...this is the first build out of 10-12 that I have had this problem. I'm at a loss, any suggestions? |
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"have positioned the block when the air was flowing the loudest" - does louder mean more flow and proper alignment? I would put pencil marks on the barrel and gas block to indicate where the holes are and align those marks. "Louder" may not be the proper term. With air flowing through the muzzle (chamber end plugged), I can rotate the gas block and actually hear when the port is partially blocked and when it is "open". |
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I had a similar problem before and the solution was counter intuitive. I also thought I was short stroking as my brass was deflecting forward and my bolt would not lock back on the last round even though it cycled fine.
By the measurements of your gas port it actually seems quite large so I think you may be over gassed and you are experiencing bolt bounce. Basically the carrier is outrunning the magazine spring. Your brass isn't actually flying forward but smashing hard against your brass deflector and then going forward. If you check your brass is it dented pretty much in the middle of the case? Is your brass deflector taking a beating? So your solution would be to try a heavier buffer. If you are using a carbine buffer move up to an H1. If you are using an H1 move up to an H2. I would give this a try since that gas port sounds like it is large. If you actually are short stroking then the only solution would to be to use a standard carbine buffer and a lower power spring. I know that Wolff makes a low power spring. |
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Not sure if I research is simply wrong, but most everything I am finding suggests that 0.093 - 0.096" is the recommended size for a 20" barrel.
Gas port size info for 5.56 barrels Barrel Length (in) Barrel Diameter (in) Distance from Muzzle (in) Min Port Size (in) Max Port Size (in) 11.5 .625 3.850 .081 .089 11.5 .750 3.850 .086 .094 14.5 .625 8.375 .063 .078 14.5 .750 8.375 .070 .086 16 .625 8.375 .063 .078 16 .750 8.375 .070 .086 20 .625 6.875 .086 .093 20 .750 6.875 .093 .096 24 .825 N/A .089 .089 This is from Colt reference material. Honestly, I don't think its an issue of overgassing. I'm not seeing any dents midway up the brass, and the brass is just lazily ejecting out the side with no real authority (roughly around 2 o'clock). Compared with my 18" MEGA build, its not nearly as aggressive to cycle, if that's even the correct term. Looking at my brass, though, I may have found another culprit. I've noticed some scratches on the brass, and definite extractor marks on the rim. I'm curious now if I just have a rough chamber that needs correcting, so I am soaking it a bit now. I don't think the BA barrels are chrome lined, at least not this one, so I am researching the best method for cleaning up the chamber. |
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Talk to BA about their gas port sizing.
I have a 18” RL gas barrel that short strokes with anything heavier than a std spring and H buffer. I almost never hear of the gas port size being the problem. Too much drag can cause short stroking. Pull back the CH, does it get unusually hard when engaging the hammer compared to working weapon? A heavy spring and/or heavy buffer may contribute to the problem. A while back it was not uncommon to hear of broken gas key screws. I would also try a different gas tube and gas block. |
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Any other suggestions? Which BA barrel is it? Is it midlength gas system or rifle length gas system? 18" RL gas systems can be finicky, because there is not much barrel left in front of the port. This is not uncommon. .093 is what I would call the absolutely minimal gas port size in an 18" and RL gas. JP, WOA, MK12 rifles all use bigger port sizes. .093 is for a 20" which has 2 extra inches of dwell time for putting pressure on the port before the bullet leaves the barrel and the system loses port pressure. There are PLENTY of threads over the years with people having 18" barrels and will just barely run with .093 gas ports. I run adjustable gas blocks, so I don't care if the factory system is a bit overgassed. In fact I prefer it. However, too much overgassing will cause excessive erosion over time. Some manufacturers run the minimal gas port size to get a reputation for being "soft shooters" which is what most people using 18" and RL gas are after. If you wanted top reliability, you'd pick 18" and ML gas. .098 would be my target port size for that barrel, up to .104 (MK12). This is made worse with a brand new rifle.... the gas rings aren't seated, the bolt carrier phosphating is rough and the upper receiver surfaces arent broken in, the action spring hasn't worn smooth, the inside of the receiver extension is still rough. This all settles in after 500 rounds or so. I like mine smooth, so I will actually polish my carrier bearing surfaces to keep from wearing my upper receiver and make the system slick, and make sure you aren't using ANY grease, only CLP, and run it wet especially on the gas rings. http://www.ar15.com/forums/t_16_4/42_Hellbenders_guides_to_Ultimate_AR15_and_or_AR10.html |
| FALARAK gave great advice on this one. Your .093" gas port is on the small side for an 18" rifle gas barrel. Are you running a rifle length receiver extension or carbine? If carbine, what spring and buffer? If you're running any of the heavy buffers and/or extra power spring, swap it out for standard carbine buffer and spring. This will be best suited for your smallish gas port. |
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Success, sort of... I cleaned up the chamber as well as I knew how, at least without being too aggressive. Put everything back together, and BCG locked back on the magazine with 1rd fired. Good so far! Loaded up 3x more rounds, fired as fast as I could pull the trigger and locked back again after the last round. So, cycling issues seemed to be solved...at least for now. But I may still have some underlying issues. I examined the brass, and I am still seeing extractor marks on the rims and some scratches on the brass. I also noted some dents on the case shoulders. Ejection pattern was around 3-4pm. At this point, I still need to do a little shooting to see how things progress. I'm going to also send an email to Ballistic Advantage, and see what they have to say about the chamber marks. Those extractor marks on Q3131 are not terribly not abnormal. Q3131 (some lots) are REALLY hot and I have seen some significant extractor marks for sure. I'd definitely want to try a few different brands and types of ammo before I made a judgement. The dented shoulders is a bit weird, but any shoulder dent would be blown out on firing, so those have to be happening on ejection. |
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The dented shoulders is a bit weird, but any shoulder dent would be blown out on firing, so those have to be happening on ejection. Yeah, it kind of confused me as well. I know you can see dents mid way up the cases on overgassed situations, where the brass is dinging off the shell deflector...but they usually land around 1 o'clock or so ahead of the ejection port. I have no idea what's causing this, but did send an email to BA. |
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For all the folks who keep talking about 18" barrels, he said it's a 20".
I would check the gas port alignment. Many people make the mistake of seating a low profile gas block against the shoulder on a barrel and it generally should be offset .040 forward. Turn the gas block around on the barrel 180 degrees and look at the gas port through the hole in the gas block. Then you will see where the block needs to be relative to the shoulder on the barrel. It needs to be centered as much as possible. |
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Quoted:
For all the folks who keep talking about 18" barrels, he said it's a 20". I would check the gas port alignment. Many people make the mistake of seating a low profile gas block against the shoulder on a barrel and it generally should be offset .040 forward. Turn the gas block around on the barrel 180 degrees and look at the gas port through the hole in the gas block. Then you will see where the block needs to be relative to the shoulder on the barrel. It needs to be centered as much as possible. No, its an 18" barrel. As far as the gas block is concerned, its aligned about as perfect as it can be. At this point, I am fairly convinced my issue is with the chamber, but really won't know for sure until I get a little more trigger time with the rifle. Good news it is cycling, bad news it is still scratching up the brass. |
| I've had good luck with AR barrels, but have you tried dropping a cartridge into the chamber and seeing if it gets stuck? On my USPSA match gun I discovered it the throat way undersized and had to have it reamed out to accommodate even factory loads, not even crazy stuff just 115gr. rounds. The test is to drop a cartridge in the chamber, tap it with your finger, and see if it easily falls out... you would be surprised... mine got stuck into the rifling so bad it required a cleaning rod to remove. Before I had the chamber reamed, the extractor was eating case rims like a mofo. Every one I collected seemed to have a dent where the extractor would not be able to pull it out very well, sometimes it didn't even extract. Havent had an issue since... May not be your issue, but it might be worth a check. |
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