So I shot 150 rounds of wolf through my AR15. The rifle functioned flawlessly. When I got home to give it a basic cleaning, I noticed there was a slight grayish film of residue on my barrel towards the end. Basically it was the same color of residue you would see on the flash hider. This was at an indoor range so I don't know if that makes a difference or not.  I have a free floated barrel with a low profile gas block. This upper was built by PSA so I haven't made any changes. This wouldn't show up too well on photo so I just ended up cleaning it off. Just worried there is some type of leak because the last time I shot this rifle , same ammo, it didn't have this film. The film was from the gas block to the muzzle. I found one other post but I would like more insight on this as one post doesn't instill confidence that this is normal. Anybody ever see this before?

The pics didn't really show it and my rail covered most of it and I wasn't gonna remove it. It wasn't a lot of like soot but just more of a slight coating but I never saw it before. I guess I'll have to check next range trip.

Post a pic of the rifle anyway, it could just be clouding under your handgaurd, if it's a long one. Don't take factory assembled stuff for granted, they're just people too. If I suspected the block for a second that handgaurd would be off immediately.

Is there a possibility that's it's just lube burning off? Oversoaking rifles in lube seems to be real popular these days.

if you really want a good answer you have to take off the rail.  You will be able to tell if its coming out of the gas block.

Probably a leak around the gas block.  You might check to make sure it's tight.  
If it is, the carbon buildup will seal off the leak in a relatively short time.  As long as the rifle is functioning correctly I wouldn't worry too much.  

I'm assuming that PSA uses steel lo-pro gas blocks.  If it's aluminum, that could explain things, too, as they tend to expand at a different rate than the barrel when heated up.  
Aluminum gas blocks that are tight when cold tend to loosen up when hot.  If that's the case, get er' hot and then carefully tighten it down some more.

 

Here the photo. You can see the clean end on the barrel and the graying part under the hand guard. I am pretty certain it is coming form the gas block as there is no residue before the gas block. This barrel only has 300 rounds through it and someone above mentioned that carbon build up might fill in the leak.

 

Yeah that could be the case of the grease as I ran these mags through pretty fast and it was really hot. I looked in the space where the gas block meets the barrel and I do not see any residue or anything so you could be right. I'll just check the barrel again when I hit the range again.

thanks

Chicken dinner!

This is my opinion too.