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Ar-15 combustion by-products on bolt

H

highscore

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I've been dealing with something thqt's been bothering me for a while. I shoot 80-100 rounds and my bolt is covered with crap, carbon, sort of green coloring... a lot of crap. I shot a match a while back with the intention of not cleaning the rifle for a Leg Match the next day and before leaving for the match, tried to pull the charging handle back and almost couldn't. Had to clean it with my toothbrush and whatever it was I had with me that weekend. I always could shoot an entire weekend and not have any issues. This is the only rifle I've had problems with. After changing the barrel it still does the same thing and most recently I thought what maybe the bore where the rings on the bolt make contact was worn, allowing the combustion to go past the rings but it still does it, even with a new bolt and carrier.

I don't think it is ammo related because we shot two rifles with the same ammo and one bolt was super clean and mine was again completely fouled. It even gets down into the trigger group. Should I just take it to the car wash on my way home from shooting?
 
frenchman said:
Gas system problems? The only thing that's plausible, to me anyway.
Click to expand...

But what, the rifle functions perfectly. Every time I've brought the subject up, everyone looks at me like it's the first time they heard of this happening. First I thought it was the Accurate 2460 I was toying with but switching back to Varget produced the same problem.
 
Check the end of the gas tube and make sure it' not cracked where the key slides over. That's my only guess.

Are the two rifles the same length gas systems?
 
xtry51 said:
Check the end of the gas tube and make sure it' not cracked where the key slides over. That's my only guess.

Are the two rifles the same length gas systems?
Click to expand...

Same gas systems, Rock Rivers and the problem child is a Bushmaster. Both are Service rifles. Bushmaster has a Krieger 6.5 twist from White Oak and the RR has a replacement RR barrel.
 
Something is not "the same" between the rifles. Gas system, buffer, bolt, ammo... Take your pics but something is quite different and only a complete tear down will identify it if you haven't found it yet.

Tear it down & take pics. Maybe its just me but people who ask for advice like this without posting pics is a pet peeve of mine.
[pics]
 
bigblue said:
Something is not "the same" between the rifles. Gas system, buffer, bolt, ammo... Take your pics but something is quite different and only a complete tear down will identify it if you haven't found it yet.

Tear it down & take pics. Maybe its just me but people who ask for advice like this without posting pics is a pet peeve of mine.
[pics]
Click to expand...

Thank you for analing my post. I will shoot it this weekend and let you anal the pics for me, sorry for being useless!
 
highscore said:
Thank you for analing my post. I will shoot it this weekend and let you anal the pics for me, sorry for being useless!
Click to expand...


I'm not specifically calling you out, this is a persistent theme and its tough to analyze this stuff without pics. It's like like I yelled at you or called you an asshat or anything lol
 
I've seen what you describe a couple times on various forums(don't ask me which I CRS) - one was assumed to be a copper/solvent residue that settled on the boat tail, the other was due to a specific powder which I can't remember of course.
 
bigblue said:
I'm not specifically calling you out, this is a persistent theme and its tough to analyze this stuff without pics. It's like like I yelled at you or called you an asshat or anything lol
Click to expand...

Hey, I called my boss a asshat today so what chance did you have! Wasn't really pissed at you but you did pick on me, at least a little. In hindsight, I guess you were trying to help. Now I just need to figure out if you hurt my reputation....
 
highscore said:
Hey, I called my boss a asshat today so what chance did you have! Wasn't really pissed at you but you did pick on me, at least a little. In hindsight, I guess you were trying to help. Now I just need to figure out if you hurt my reputation....
Click to expand...

I gave you a positive rep so the only way I hurt it is if you were trying to get ban hammered...
 
highscore said:
Shot a match a few weeks ago using the same ammo for both rifles. Bushmaster was much more fouled than the RR. When I last shot the Accurate 2460 is did the same thing.
Click to expand...

OK. so that rules out ammo.

Here is a trick that you may want to try. The visual is terrible, and people are going to laugh, but it is a useful way to compare the gas systems.

Mount the gun in a mag block in a vise.
Chamber an EMPTY piece of fired brass with the primer still in place.
put your mouth over the muzzle and blow.

Do the same thing for the second rifle and compare them.

Someone taught me this years ago as a means to confirm gas block placement. It works. All kidding aside.

This test simultaniously tests
1) gas block location.
2) gas tube fit in block
3) gas tube fit with BCG gas key
4) BCG gas key seal wit BCG
5) Bolt rings

If you find a big difference, then try switching bolt carrier groups and see if the difference stays with the rifle or with the BCG.
If the difference changes with the BCG, then the difference is in the BCG. If the difference remains with the rifle then its something in the gas system forward of the BCG.

Don
 
Last edited:
dcmdon said:
OK. so that rules out ammo.

Here is a trick that you may want to try. The visual is terrible, and people are going to laugh, but it is a useful way to compare the gas systems.

Mount the gun in a mag block in a vise.
Chamber an EMPTY piece of fired brass with the primer still in place.
put your mouth over the muzzle and blow.

Do the same thing for the second rifle and compare them.

Someone taught me this years ago as a means to confirm gas block placement. It works. All kidding aside.

This test simultaniously tests
1) gas block location.
2) gas tube fit in block
3) gas tube fit with BCG gas key
4) BCG gas key seal wit BCG
5) Bolt rings

If you find a big difference, then try switching bolt carrier groups and see if the difference stays with the rifle or with the BCG.
If the difference changes with the BCG, then the difference is in the BCG. If the difference remains with the rifle then its something in the gas system forward of the BCG.

Don
Click to expand...

So what happens when you suck on the barrel?
 
dcmdon said:
Mount the gun in a mag block in a vise.
Chamber an EMPTY piece of fired brass with the primer still in place.
put your mouth over the muzzle and blow.

Do the same thing for the second rifle and compare them.

Someone taught me this years ago as a means to confirm gas block placement. It works. All kidding aside.
Click to expand...

Nice. You could probably use a compressed air nozzle too.
 
Tallen said:
Nice. You could probably use a compressed air nozzle too.
Click to expand...

I was going to post that, but then I couldn't figure out how you would be able to determine if there was a pressure difference between the two. Maybe start by turning the air pressure super low, and slowly increasing it until you find out which pressure is required to move the action.
 
How's the brass coming out of the 2 guns using the same ammo?
Something is different. My guess maybe chamber is looser on the problem child ? Or possible, the timing is off and the bolt is unlocking before the pressure in chamber completely bleeds off?
Is it possible the bullets are getting heavily scored/ scraped allowing some copper residue at a higher rate down through the gas system?
 
mac1911 said:
How's the brass coming out of the 2 guns using the same ammo?
Something is different. My guess maybe chamber is looser on the problem child ? Or possible, the timing is off and the bolt is unlocking before the pressure in chamber completely bleeds off?
Is it possible the bullets are getting heavily scored/ scraped allowing some copper residue at a higher rate down through the gas system?
Click to expand...

Just to expand on Mac's thoughts on timing. There are several things that affect timing. The biggies are
1) Gas port size
2) Buffer spring shape
3) Buffer mass

Which leads me to another thought. Compare buffer spring strength and free length. Compare buffer weight. My guess is that this won't go anywhere since there isn't a lot of variation among rifle systems. But its worth a shot if only to eliminate some possible differences.

If you do find a difference, swap the spring/buffer and see if the problem stays with the rifle or moves with the spring/buffer.

By the way, my blowing on the end of the barrel trick is a way to flush out different sized gas ports.

Don
 

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