Heavy carbon build up only after gas port on AR15

[PigButtons]

I've not seen this on any of my other ARs but I bought a used upper and like most used stuff from misc auction buys it was in need of a cleaning. Now that I have a bore scope I wanted to see how bad the task was going to be. I was surprised to find crusty hard to remove heavy carbon only after the gas port in the barrel and before the gas port it was very clean.

Has anyone else seen this, or can you explain why it might happen?

It is a 24" heavy, bull, chromoly, 1:8 twist, Unit One, with a massive handguard, tripod, and fore end hand stop like on a position target rifle. Standard 5.56.

 

[Big Mc]

I've not seen this on any of my other ARs but I bought a used upper and like most used stuff from misc auction buys it was in need of a cleaning. Now that I have a bore scope I wanted to see how bad the task was going to be. I was surprised to find crusty hard to remove heavy carbon only after the gas port in the barrel and before the gas port it was very clean.

Has anyone else seen this, or can you explain why it might happen?

It is a 24" heavy, bull, chromoly, 1:8 twist, Unit One, with a massive handguard, tripod, and fore end hand stop like on a position target rifle. Standard 5.56.

I think the other guy did not clean it properly. Just my two cents Tommy Mc

 

[akajun]

sure your not seeing the "comet tail" that is common with gas ports? its not carbon its actual wear in the barrel from gas jetting out.

 

[Catfur]

Has anyone else seen this, or can you explain why it might happen?

A potential explanation might be that a sudden drop in pressure after the gas port (due to the gas being vented) causes the temp to drop and carbon to precipitate out onto the barrel surfaces.

 

[PigButtons]

sure your not seeing the "comet tail" that is common with gas ports? its not carbon its actual wear in the barrel from gas jetting out.

I'd never heard of "comet trail" so I'll watch for that as well. But this is carbon and is responding slowly to BoreTech C4 and a nylon brush. 4 hours into cleaning it and about 2/3 of the carbon is gone. Now another 4 hours to get the last 2/3rds.

 

[PigButtons]

A potential explanation might be that a sudden drop in pressure after the gas port (due to the gas being vented) causes the temp to drop and carbon to precipitate out onto the barrel surfaces.

I thought about this as well but had never heard of or seen it before. This is the first 1" bull AR barrel I've owned so the temperature dynamic in the barrel could certainly be the cause. Thanks for the confirmation that it is at least plausible.

 

[jbarron]

Might have been a burr on the gas port originally that helped the build up and it just never got cleaned properly.

 

[gdbleb]

"comet tail" is an apt description. I always thought it was both carbon and erosion.

 

[jelenko]

If C4 is having an effect, try letting it sit over night. It'll soften a bunch of the carbon.

Letting C4 sit for a while dramatically reduces the number of strokes with the cleaning rod.

 

[PigButtons]

If C4 is having an effect, try letting it sit over night. It'll soften a bunch of the carbon.

Letting C4 sit for a while dramatically reduces the number of strokes with the cleaning rod.

Thanks for the tip. Several six hour soaking periods and there are only a few small spots of carbon left.

And the really great news is that the barrel shoots 1" groups @ 100 with M855. Can't wait to see what it'll do with some tuning and more precise loads.