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Posted: 8/3/2014 1:26:52 PM EDT
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Put together another AR and went to the range again today after cleaning/lubing and many other checks. It's short stroking and it's driving me nuts -Will not bolt hold open on last round. Basically, I am manually cycling the rifle. -Don't see anything wrong with the way it ejects a round -Replaced upper with some random cheap PSA upper - worked fine. So, I know the issue is upper related. Or so it seems. -Replaced the BCG, with a standard Bushmaster - still the same issue -Verified BCG moves freely with little effort and not binding with gas tube -Pressure tested the gas system using the following technique with the upper removed from lower:
There is some rubbing that seems a bit abnormal with the charging handle against the inside of the upper receiver (see pics- top of upper receiver). Maybe slightly out of spec upper? I see these marks in other AR's, but this one seems a bit excessive - maybe I'm over analyzing  . It doesn't seem sticky - pulling charging handle all the way back has the same resistance as my other AR's. I'm not feeling any binding/hesitation that's different than my other AR's. And, the BCG moves very freely inside the upper with the charging handle in place. Obviously, I cannot test the pressures that occur in a real firing. So, I suppose it could still be a gas port size issue or some other type of gas system issue. I'm hoping someone on here might point me in the right direction. Pics of my headache rifle:       |
| All the wear inside the upper looks normal to me. My best guesses would be 1-gas block is misaligned whether it's too far forward or backward (looks to be at 12:00 as far as side to side) allowing only some of the gas to get into the block. 2- too much is leaking where you spotted it where the tube and block meet, or 3- (the least likely) gas port drilled to wrong size |
Confirm that your gas tube is fully-inserted into the gas block. In photo #3, it looks like the end of the installed tube is too far back: when correctly-installed, the end of the tube inside the upper receiver should be more or less aligned with the raised line on the outside of the receiver swell, left side. Photo #5 seems to show the gas tube roll pin not installed flush, another clue that something's off with the gas tube. HTH. 
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What are your weapon's specs?
i.e. barrel length, gas system used, barrel's gas port diameter, action spring used and spring weight, buffer weight, extractor spring used (# of coils), extractor spring insert?, extractor spring o-ring?, inside diameter of gas tube (should be 0.120") |
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Quoted: Confirm that your gas tube is fully-inserted into the gas block. In photo #3, it looks like the end of the installed tube is too far back: when correctly-installed, the end of the tube inside the upper receiver should be more or less aligned with the raised line on the outside of the receiver swell, left side. Photo #5 seems to show the gas tube roll pin not installed flush, another clue that something's off with the gas tube. HTH. |
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Quoted: What are your weapon's specs? i.e. barrel length, gas system used, barrel's gas port diameter, action spring used and spring weight, buffer weight, extractor spring used (# of coils), extractor spring insert?, extractor spring o-ring?, inside diameter of gas tube (should be 0.120") Buffer Spring: Ade Advanced Optics Ar15 M4 .223 5.56 Commercial Spec Buffer Spring - http://www.bulletproofvestshop.com/ade-advanced-optics-ar15-m4-223-5-56-commercial-spec-buffer-spring/ Carbine Gas tube: http://www.strikeindustries.com/shop/index.php/rifle-accessories/ar/gas-tubes/ar-15-m16-gas-tube-stainless-steel-silver-color.html Spikes T2 Buffer: http://www.spikestactical.com/lower-parts-spikes-stt2-heavy-buffer-p-201.html Barrel length = 16" - Adams Arms Voodoo Innovations 16" Melonite Carbine M4 Barrel Gas Block: Vltor Low Profile Gas Block, Clamp Mounted, .750 Bore |
Damn it... For the heck of it, I looked up the reviews on the barrel 'just in case' there are known issues.  Take a look at the reviews (not where I purchased it from) on Midway:http://www.midwayusa.com/product/616771/voodoo-innovations-barrel-ar-15-556x45mm-nato-m4-contour-carbine-gas-port-1-in-7-twist-16-chrome-moly-black-melonite-finish#ReviewHeader Looks like some people have reported incorrect gas port size. I'm no expert builder - are these issues common?? I mean, do I got to start checking the diameter of the inside of barrels now as the diameter of the bore could be too small?? I will check mine for sure when I get back - with my luck, this is the issue. So much for meloniting the barrel when I have to potentially cut into the steel. Good thing there's no corrosive 5.56  |
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Drilling a gas port should be the VERY LAST POSSIBLE step.
And the change you made is way too much. You increased the gas port area by 68%. If it was ejecting but not locking back you didn't need anywhere near that much additional energy. One contributing factor to your issue is improper assembly - your gas block is installed too far to the rear. You have to leave the space that would otherwise be occupied by a handguard ring, about .045 ~ .050. It does not install against the shoulder. The off-center gas port disrupts the gas flow and can reduce energy to the operating system, or in more extreme cases even partially obstruct the gas port. |
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Quoted:
Drilling a gas port should be the VERY LAST POSSIBLE step. And the change you made is way too much. You increased the gas port area by 68%. If it was ejecting but not locking back you didn't need anywhere near that much additional energy. One contributing factor to your issue is improper assembly - your gas block is installed too far to the rear. You have to leave the space that would otherwise be occupied by a handguard ring, about .045 ~ .050. It does not install against the shoulder. The off-center gas port disrupts the gas flow and can reduce energy to the operating system, or in more extreme cases even partially obstruct the gas port. Good eye Gamma. I figured he would've checked this after I recommended it, but I don't think he told us if he did or not before drilling out the gas port lol |
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Quoted: Good eye Gamma. I figured he would've checked this after I recommended it, but I don't think he told us if he did or not before drilling out the gas port lol Quoted: Quoted: Drilling a gas port should be the VERY LAST POSSIBLE step. And the change you made is way too much. You increased the gas port area by 68%. If it was ejecting but not locking back you didn't need anywhere near that much additional energy. One contributing factor to your issue is improper assembly - your gas block is installed too far to the rear. You have to leave the space that would otherwise be occupied by a handguard ring, about .045 ~ .050. It does not install against the shoulder. The off-center gas port disrupts the gas flow and can reduce energy to the operating system, or in more extreme cases even partially obstruct the gas port. Good eye Gamma. I figured he would've checked this after I recommended it, but I don't think he told us if he did or not before drilling out the gas port lol Update: I took it to the range yesterday and fired 100 rounds... No issue, problem solved. I just never bothered to check the port diameter when buying a barrel. Now I know.
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Quoted: That's good to hear bro! It's not overgassed? So, I wouldn't be able to tell you if it's over-gassed - From what I read, the recoil would be noticeably higher. However, I hardly ever notice the difference between the recoil of one AR vs another. I'm just glad it cycles reliably. I guess I can buy any play with an adjustable block and tweak it just right, not sure if it's worth it as I don't shoot competitive. Thanks for all the replies! |
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Quoted:
Not sure, but I'm no pro So, I wouldn't be able to tell you if it's over-gassed - From what I read, the recoil would be noticeably higher. However, I hardly ever notice the difference between the recoil of one AR vs another.
I'm just glad it cycles reliably. I guess I can buy any play with an adjustable block and tweak it just right, not sure if it's worth it as I don't shoot competitive. Thanks for all the replies! Quoted:
Quoted:
That's good to hear bro! It's not overgassed? So, I wouldn't be able to tell you if it's over-gassed - From what I read, the recoil would be noticeably higher. However, I hardly ever notice the difference between the recoil of one AR vs another.
I'm just glad it cycles reliably. I guess I can buy any play with an adjustable block and tweak it just right, not sure if it's worth it as I don't shoot competitive. Thanks for all the replies! Easiest way to judge overgassing issues is to see what position the brass ejects at. (1:00 position being overgassed) which isn't a devastating issue, but will wear out your internals faster, is more fun to shoot tho. To me at least lol |
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Area of the port goes up with the square of the size. Much more than the increase in diameter. Quoted:
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Nice to hear, on another note isn't .061 to .078 more like a 28% increase? Still a big step up in the porting department. Area of the port goes up with the square of the size. Much more than the increase in diameter. Thanks Gamma, love edumacation and didn't think about the squaring of the size. Walking/talking encyclopedia you are. 
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| Yikes, .078 is pretty large for a 16" barrel with carbine gas system. .063 is pretty much the norm for that configuration. .078 would be great if it was a 16" middy however. I probably would have tried a lighter buffer before drilling the port, but at least it sounds like you got it running. Chances are it is overgassed though. |
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Quoted:
Thanks Gamma, love edumacation and didn't think about the squaring of the size. Walking/talking encyclopedia you are. Quoted:
Quoted:
Quoted:
Nice to hear, on another note isn't .061 to .078 more like a 28% increase? Still a big step up in the porting department. Area of the port goes up with the square of the size. Much more than the increase in diameter. Thanks Gamma, love edumacation and didn't think about the squaring of the size. Walking/talking encyclopedia you are. Agreed. I think I learned everything that I do know about AR's pretty much from lurking around reading gamma and danos posts and others as knowledgeable about the platform as them before I even made an account here, including the archives and Danos posts in the m4carbine archives (took me years before I actually made a gmail account so I could register here and there, only had yahoo) my hat is off to you guys. (My sweet BCM hat) 
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