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Posted: 12/14/2013 8:16:07 PM EDT
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Took the rifle I built for my dad to the range today to test it out and sight it in before Christmas.
I was using PVRI Partizan .55 grain factory ammo. On some shots, the bolt was not stripping a round from the magazine, and on others when it did strip the round, it didn't have enough inertia to send it home to the chamber. It seemed like pretty obvious short stroking. It is a brand new gas tube and a low profile gas block. I brought it home and pulled the handguard and gas block just to be sure everything looked gtg. Seemed alright. I also checked the gas key, no visible signs of leakage, and the allen screws seemed sufficiently tight (I don't currently have a torque wrench). The rifle has a DPMS carbine buffer and buffer spring on a carbine length gas system 16" barrel. Also the BCG is the standard AIM Surplus brand, not the Nickel Boron one. It seems like it is obviously short stroking, am I correct in that assumption? If so, is there something I'm not thinking of to check? Is it possible I just need to shoot it more to break it in some? Is there something else I can do to speed the process? |
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The gas block allen bolts should set with red loctite. If you need to remove the block down the road, use a old allen wrench in the bolts and heat the wrench up with a torch to transfer heat to the allen bolts to break down the loctite before removing them with a new wrench,
Next we get into correctly cleaning (including scrubbing the chamber with a chamber brush and CLP by hand) and correctly CLP lubing the upper bearing areas with CLP before the rifle is shot. Lastly, if you are having problems and the above conditions have been meet, then try other USA brass case ammo to weed out if it just the ammo the problem alone. |
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I had the same issues, mine ended up being the BCG leaking around the gas key. Only figured this out after checking everything else a dozen times (literally tearing it apart 12 times and rebuilding).
Also, is there any undue resistance when installing the BCG? Rubbing on the gas tube? Rubbing somewhere else? Were you running it wet? |
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I had the same issues, mine ended up being the BCG leaking around the gas key. Only figured this out after checking everything else a dozen times (literally tearing it apart 12 times and rebuilding). Also, is there any undue resistance when installing the BCG? Rubbing on the gas tube? Rubbing somewhere else? Were you running it wet? the BCG cycles smoothly, one thing I haven't checked is the bolt gas rings to make sure they aren't aligned. I will check that to be sure. I guess a good pressure check of the BCG is in order. |
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The gas block allen bolts should set with red loctite. If you need to remove the block down the road, use a old allen wrench in the bolts and heat the wrench up with a torch to transfer heat to the allen bolts to break down the loctite before removing them with a new wrench, Next we get into correctly cleaning (including scrubbing the chamber with a chamber brush and CLP by hand) and correctly CLP lubing the upper bearing areas with CLP before the rifle is shot. Lastly, if you are having problems and the above conditions have been meet, then try other USA brass case ammo to weed out if it just the ammo the problem alone. I will set the allen bolts with red loctite. The chamber was thoroughly scrubbed with a chamber brush. I used a standard gun oil to lube, not CLP, but I could lube with CLP and try it again. I shot the same ammo out of my rifle with no issues, so I don't think it is under pressured. |
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the BCG cycles smoothly, one thing I haven't checked is the bolt gas rings to make sure they aren't aligned. I will check that to be sure. I guess a good pressure check of the BCG is in order. Quoted:
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I had the same issues, mine ended up being the BCG leaking around the gas key. Only figured this out after checking everything else a dozen times (literally tearing it apart 12 times and rebuilding). Also, is there any undue resistance when installing the BCG? Rubbing on the gas tube? Rubbing somewhere else? Were you running it wet? the BCG cycles smoothly, one thing I haven't checked is the bolt gas rings to make sure they aren't aligned. I will check that to be sure. I guess a good pressure check of the BCG is in order. Just to be clear here this has proven to be a non issue. Most guns will run with only one ring in place and many will run with no rings. The easiest way to check the gas rings to simply push the bolt into the carrier and hold the carrier vertically and see if the rings offer enough resistance to see if the bolt will fall from the carrier. They should be tight enough to prevent this. Short stroking can be caused by weak gas or dragging components. Or a combination of both. |
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If and when you do get through my other post above, and find that it's till having problems with known good USA brass case ammo, then you can start checking for gas leaks.
To check for leaks, use a tube on the end of the gas tube inside the receiver, and with the gas block to barrel/ gas tube to block areas sprayed with CLP pressurize the tube to see if you can see leaking at these junction areas. Also to note, same time you are checking for block leaks, you can check for a clean tube/block/barrel port gas alignment, which should be allowing air to flow through the gas tube, and out of the barrel bore via the barrel gas port. On the B/C, CLP at the base of the key all around, and hold the bolt in while pressurizing the key with compressed air. There are area on the B/C that will leak, but between the key to top of carrier is not one of them. In both pressure cases, you should not have a leak from the gas tube to gas block, the gas block to the barrel, nor the key to the top of the carrier. Also, with just the carrier and key alone (no bolt) dry fit it into the upper receiver to check the alignment of the key to gas tube. If you get binding of the tube not being centered to the key, then adjust the gas tube to center it with the carrier key. |
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Okay, so I followed procedures above to check for leaks in the gas system, and I did not find any. I took the rifle back out and fired four kinds of ammo. I had short stroking issues with pvri, PMC, reloads (24 grains 2230, 55gr FMJ 2.250 oal slight crimp) and wolf gold.
The wolf gold is the only ammo that didn't short stroke at all with 40 rounds fired. The others ranged from short striking nearly every round to every 4-5 rounds. What say you AR15 experts? ETA: I also brought a BCG that I know functions flawlessly and tested the upper with it for each of the three ammos that were short stroking. It mirrored the results of the BCG that lives in this upper. |
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Rifle correctly cleaned (including using a chamber brush with CLP in the chamber before running dry patches down the barrel) and the upper bearing areas lubed on the heaver side with CLP?
If yes, then shotgun the upper open, and take a look at the tail of the trigger and hammer for a denting or while line telling you that the hammer is wedging between the FCG and the bottom of the receiver. If you have another lower, then drop the upper on it for a test drive to weed out the lower being the problem. From there and still having problems with the upper on another lower, then need to know the barrel gas port size if you are sure that there is not other binding/leakage/blockage going on in the gas system. |
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Okay, so I followed procedures above to check for leaks in the gas system, and I did not find any. I took the rifle back out and fired four kinds of ammo. I had short stroking issues with pvri, PMC, reloads (24 grains 2230, 55gr FMJ 2.250 oal slight crimp) and wolf gold. The wolf gold is the only ammo that didn't short stroke at all with 40 rounds fired. The others ranged from short striking nearly every round to every 4-5 rounds. What say you AR15 experts? ETA: I also brought a BCG that I know functions flawlessly and tested the upper with it for each of the three ammos that were short stroking. It mirrored the results of the BCG that lives in this upper. xphiya, Funny you mention about PMC. One of my AR''s had short stroked on PMC Bronze BUT trying some old PMC Target or Military everything worked fine. Try if you haven't oiling your recoil spring tube more. I did it to mine and perhaps the spring is new or need to be broken in a little. My opinion....PMC Bronze is weak. Impala |
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Rifle correctly cleaned (including using a chamber brush with CLP in the chamber before running dry patches down the barrel) and the upper bearing areas lubed on the heaver side with CLP? If yes, then shotgun the upper open, and take a look at the tail of the trigger and hammer for a denting or while line telling you that the hammer is wedging between the FCG and the bottom of the receiver. If you have another lower, then drop the upper on it for a test drive to weed out the lower being the problem. From there and still having problems with the upper on another lower, then need to know the barrel gas port size if you are sure that there is not other binding/leakage/blockage going on in the gas system. I took a look at the lower/FCG, no signs of binding. The barrel on the upper is found here. I checked the gas port hole for blockage with a small pin, no signs. After Christmas probably I will take it out and try it on my other lower to eliminate that as being the issue. |
| Before you changed the gas block I assume the rifle functioned correctly. When you change to the low pro gas block did you reinstall the handguard retaining ring? Your gas port may not be aligned correctly, Also what brand of gas block is it? If it's something you got off of ebay, gun show or a no name brand block it may not be drilled correctly or it may have metal shaving blocking/limiting the flow. If anything replace the original gas block on any try tool shoot it again. Also is your bolt/cam key oiled? |
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Before you changed the gas block I assume the rifle functioned correctly. When you change to the low pro gas block did you reinstall the handguard retaining ring? Your gas port may not be aligned correctly, Also what brand of gas block is it? If it's something you got off of ebay, gun show or a no name brand block it may not be drilled correctly or it may have metal shaving blocking/limiting the flow. If anything replace the original gas block on any try tool shoot it again. Also is your bolt/cam key oiled? I built the rifle from scratch, never replaced the gas block. It is a YHM low profile block, the same one I have on my other rifle, and appears to be machined just fine. One thing I thought MAY be an issue is if the barrel and/or gas block aren't exactly to spec at the gas port... like for instance if the barrel was .748 instead of .750. I want to recheck for leaks there, just to be sure. |
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I built the rifle from scratch, never replaced the gas block. It is a YHM low profile block, the same one I have on my other rifle, and appears to be machined just fine. One thing I thought MAY be an issue is if the barrel and/or gas block aren't exactly to spec at the gas port... like for instance if the barrel was .748 instead of .750. I want to recheck for leaks there, just to be sure. Quoted:
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Before you changed the gas block I assume the rifle functioned correctly. When you change to the low pro gas block did you reinstall the hand guard retaining ring? Your gas port may not be aligned correctly, Also what brand of gas block is it? If it's something you got off of ebay, gun show or a no name brand block it may not be drilled correctly or it may have metal shaving blocking/limiting the flow. If anything replace the original gas block on any try tool shoot it again. Also is your bolt/cam key oiled? I built the rifle from scratch, never replaced the gas block. It is a YHM low profile block, the same one I have on my other rifle, and appears to be machined just fine. One thing I thought MAY be an issue is if the barrel and/or gas block aren't exactly to spec at the gas port... like for instance if the barrel was .748 instead of .750. I want to recheck for leaks there, just to be sure. Might want to pull the gas block and check. Start off by putting a pencil mark on the shoulder of the barrel dead center of the gas tube. This way you know the oritation of the block to the barrel. Use a old allen wrench in the allen bolts, heat the wrench up with a torch until you smell a sweet smell (loctite bond breaking down), then switch over to an new allen wrench to remove the screws to pull the block. With the block off, take a look at the burn ring around the barrel gas port see if you had the block gas passage centered on the barrel port. The block gas passage is about 4 times the size of a gas port, so there is some wiggle room when you install it. Next, try to get a size of the gas port via a drill bit. Not sure if you ever stated the barrel, but here is a drill bit conversion size chart if needed. Use the drill bit as a gauge, and do not open the gas port any more open at this time until we know the barrel (Barrel length, and OD size of the barrel at the gas port). http://www.gearhob.com/eng/design/drill_eng.htm Now since you have the gas block out, and the screws out of the gas block, look straight up the back thread channel of the gas block. This channel is used for both the rear screw, and to drill the gas passage. Here you are checking to make sure that the side opening of the gas tube is centered in the passage opening ID of the block. If you can not see the tube side openning in the channel, they chances are you either did not install the gas tube far enough into the block before the cross pin was installed, or you installed the tube upside down in the block. If you do see the tube side opening in the channel, but the block passage to tube side opening is slightly off, you can use a drill but up through the thread channel to enlarge the tube side opening to match the passage channel. Now granted that we are waiting back for the barrel gas port size (and barrel length and OD diameter at the gas passage on the barrel), when you do reinstall the gas block with gas tube, you have the barrel shoulder reference marks for any adjustments if needed. Also, once the gas block has been reinstalled (remember to use Red loctite on the allen bolts so they do not loosen up down the line), you need to use the carrier with key (minus bolt) in the upper receiver to confirm the alignment of the gas tube to the carrier key. If you can not pull the hand guards to tweak the gas tube over the barrel with the block mounted, then it might to best to do some temp gas block mountings without loctite first so you can pull the block/tube, give it a quick tweak, then temp reinstall it when needed. In the very end, you could do a quick tweak of the very end of the gas block within the receiver, but you want to get the gas tube closely tweaked before hand. As for gas port sizes, here is the general start point. But to point out, do not increase the gas port larger than that is listed to band-aid for a problem in the rifle elsewhere. I state this since if you have a barrel that is head spacing tighter that it should be, this alone will cause short stroking problems that no enlarging of the gas port size is going to resolve. 11.5" barrel, .625: dia., 3.850" from muzzle; Port sizes from .081" to .089" 11.5" barrel, .750" dia., 3.850" from muzzle Port sizes from .086" to .094 14" barrel, .625" dia., 8.375" from muzzle; Port sizes from .063" to .078" 14" barrel, .750 dia., 8.375" from muzzle; Port sizes from .070" to .086" 16" barrel, .625" dia., 8.375" from muzzle; Port sizes from .063" to 078" 16" barrel, .750" dia., 8.375" from muzzle; Port sizes from .070" to .086" 20" barrel, .625" dia., 8.375" from muzzle; Port sizes from .086" to .093" 20" barrel, .750" dia., 6.875" from muzzle; Port sizes from .093" to .096" 24" barrel, .825" dia., Port size .089" |
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Might want to pull the gas block and check. Start off by putting a pencil mark on the shoulder of the barrel dead center of the gas tube. This way you know the oritation of the block to the barrel. Use a old allen wrench in the allen bolts, heat the wrench up with a torch until you smell a sweet smell (loctite bond breaking down), then switch over to an new allen wrench to remove the screws to pull the block. With the block off, take a look at the burn ring around the barrel gas port see if you had the block gas passage centered on the barrel port. The block gas passage is about 4 times the size of a gas port, so there is some wiggle room when you install it. Next, try to get a size of the gas port via a drill bit. Not sure if you ever stated the barrel, but here is a drill bit conversion size chart if needed. Use the drill bit as a gauge, and do not open the gas port any more open at this time until we know the barrel (Barrel length, and OD size of the barrel at the gas port). http://www.gearhob.com/eng/design/drill_eng.htm Now since you have the gas block out, and the screws out of the gas block, look straight up the back thread channel of the gas block. This channel is used for both the rear screw, and to drill the gas passage. Here you are checking to make sure that the side opening of the gas tube is centered in the passage opening ID of the block. If you can not see the tube side openning in the channel, they chances are you either did not install the gas tube far enough into the block before the cross pin was installed, or you installed the tube upside down in the block. If you do see the tube side opening in the channel, but the block passage to tube side opening is slightly off, you can use a drill but up through the thread channel to enlarge the tube side opening to match the passage channel. Now granted that we are waiting back for the barrel gas port size (and barrel length and OD diameter at the gas passage on the barrel), when you do reinstall the gas block with gas tube, you have the barrel shoulder reference marks for any adjustments if needed. Also, once the gas block has been reinstalled (remember to use Red loctite on the allen bolts so they do not loosen up down the line), you need to use the carrier with key (minus bolt) in the upper receiver to confirm the alignment of the gas tube to the carrier key. If you can not pull the hand guards to tweak the gas tube over the barrel with the block mounted, then it might to best to do some temp gas block mountings without loctite first so you can pull the block/tube, give it a quick tweak, then temp reinstall it when needed. In the very end, you could do a quick tweak of the very end of the gas block within the receiver, but you want to get the gas tube closely tweaked before hand. As for gas port sizes, here is the general start point. But to point out, do not increase the gas port larger than that is listed to band-aid for a problem in the rifle elsewhere. I state this since if you have a barrel that is head spacing tighter that it should be, this alone will cause short stroking problems that no enlarging of the gas port size is going to resolve. 11.5" barrel, .625: dia., 3.850" from muzzle; Port sizes from .081" to .089" 11.5" barrel, .750" dia., 3.850" from muzzle Port sizes from .086" to .094 14" barrel, .625" dia., 8.375" from muzzle; Port sizes from .063" to .078" 14" barrel, .750 dia., 8.375" from muzzle; Port sizes from .070" to .086" 16" barrel, .625" dia., 8.375" from muzzle; Port sizes from .063" to 078" 16" barrel, .750" dia., 8.375" from muzzle; Port sizes from .070" to .086" 20" barrel, .625" dia., 8.375" from muzzle; Port sizes from .086" to .093" 20" barrel, .750" dia., 6.875" from muzzle; Port sizes from .093" to .096" 24" barrel, .825" dia., Port size .089" Quoted:
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Before you changed the gas block I assume the rifle functioned correctly. When you change to the low pro gas block did you reinstall the hand guard retaining ring? Your gas port may not be aligned correctly, Also what brand of gas block is it? If it's something you got off of ebay, gun show or a no name brand block it may not be drilled correctly or it may have metal shaving blocking/limiting the flow. If anything replace the original gas block on any try tool shoot it again. Also is your bolt/cam key oiled? I built the rifle from scratch, never replaced the gas block. It is a YHM low profile block, the same one I have on my other rifle, and appears to be machined just fine. One thing I thought MAY be an issue is if the barrel and/or gas block aren't exactly to spec at the gas port... like for instance if the barrel was .748 instead of .750. I want to recheck for leaks there, just to be sure. Might want to pull the gas block and check. Start off by putting a pencil mark on the shoulder of the barrel dead center of the gas tube. This way you know the oritation of the block to the barrel. Use a old allen wrench in the allen bolts, heat the wrench up with a torch until you smell a sweet smell (loctite bond breaking down), then switch over to an new allen wrench to remove the screws to pull the block. With the block off, take a look at the burn ring around the barrel gas port see if you had the block gas passage centered on the barrel port. The block gas passage is about 4 times the size of a gas port, so there is some wiggle room when you install it. Next, try to get a size of the gas port via a drill bit. Not sure if you ever stated the barrel, but here is a drill bit conversion size chart if needed. Use the drill bit as a gauge, and do not open the gas port any more open at this time until we know the barrel (Barrel length, and OD size of the barrel at the gas port). http://www.gearhob.com/eng/design/drill_eng.htm Now since you have the gas block out, and the screws out of the gas block, look straight up the back thread channel of the gas block. This channel is used for both the rear screw, and to drill the gas passage. Here you are checking to make sure that the side opening of the gas tube is centered in the passage opening ID of the block. If you can not see the tube side openning in the channel, they chances are you either did not install the gas tube far enough into the block before the cross pin was installed, or you installed the tube upside down in the block. If you do see the tube side opening in the channel, but the block passage to tube side opening is slightly off, you can use a drill but up through the thread channel to enlarge the tube side opening to match the passage channel. Now granted that we are waiting back for the barrel gas port size (and barrel length and OD diameter at the gas passage on the barrel), when you do reinstall the gas block with gas tube, you have the barrel shoulder reference marks for any adjustments if needed. Also, once the gas block has been reinstalled (remember to use Red loctite on the allen bolts so they do not loosen up down the line), you need to use the carrier with key (minus bolt) in the upper receiver to confirm the alignment of the gas tube to the carrier key. If you can not pull the hand guards to tweak the gas tube over the barrel with the block mounted, then it might to best to do some temp gas block mountings without loctite first so you can pull the block/tube, give it a quick tweak, then temp reinstall it when needed. In the very end, you could do a quick tweak of the very end of the gas block within the receiver, but you want to get the gas tube closely tweaked before hand. As for gas port sizes, here is the general start point. But to point out, do not increase the gas port larger than that is listed to band-aid for a problem in the rifle elsewhere. I state this since if you have a barrel that is head spacing tighter that it should be, this alone will cause short stroking problems that no enlarging of the gas port size is going to resolve. 11.5" barrel, .625: dia., 3.850" from muzzle; Port sizes from .081" to .089" 11.5" barrel, .750" dia., 3.850" from muzzle Port sizes from .086" to .094 14" barrel, .625" dia., 8.375" from muzzle; Port sizes from .063" to .078" 14" barrel, .750 dia., 8.375" from muzzle; Port sizes from .070" to .086" 16" barrel, .625" dia., 8.375" from muzzle; Port sizes from .063" to 078" 16" barrel, .750" dia., 8.375" from muzzle; Port sizes from .070" to .086" 20" barrel, .625" dia., 8.375" from muzzle; Port sizes from .086" to .093" 20" barrel, .750" dia., 6.875" from muzzle; Port sizes from .093" to .096" 24" barrel, .825" dia., Port size .089" Thank you for the info. The barrel is linked above, but for reference it is a 16" barrel with a carbine length gas system and a diameter at the port of .750. So based on that the port size should be between .070 and .086 correct? |
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Thank you for the info. The barrel is linked above, but for reference it is a 16" barrel with a carbine length gas system and a diameter at the port of .750. So based on that the port size should be between .070 and .086 correct? Gas port sizes will vary among manufacturers and sometimes the particular model of barrel. When teaching armorer courses, we see lots of barrel configurations from numerous makers, so we get to gauge and inspect them. On 16" barrels with a carbine length gas system will see gas ports that run from .057" to .089", with an average of .062". CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 |
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To point it out since Sully brought it up with small gas ports, Colt tends to port the M- 4 barrels on the small side, due to them being designed for anything from M-193 to Mk-262 ammo (that the gun may be suppressed).
On civilian rigs that you are going to run 223/suplus ammo, the smaller gas port will cause nothing but short stroking problems. Simply, neither Sully or myself is advising that you open the gas port up yet until you report your findings, but at the same time, keep in mind that there are rifle barrel out there with smaller gas ports (which are designed to all them to run hotter ammo's suppressed), as well as some barrel with tight chambers for civilian 223 ammo that may require a slightly larger gas port to run instead. |
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