A friend of mine has a rifle that jams quite a bit.  I offered to help him out but can't figure it out.  I used good mags and ammo -helped him clean and lube it- checked rifle over for loose, bent or ill-fitting parts.  Nothing stood out.  It still jams about once per 20 rd mag, and becomes a single shot after about 200 rounds.  There did seem to be a fair amount of carbon building up at the front end of the gas tube and under the front site.  Is that normal, or is it leaking a bit?  Is there any way to verify without taking it apart completely?  

after a lot of shooting, its not unusual to see some evidence of carbon at the front sight.  but thats usually after A LOT of rounds - like the 200 or so plus.  i see a little sometimes. if there looks to be alot, i'd have someone who has fooled with the gas tubes to look at it.  it may be loose or may need another.  they're pretty cheap.  what kind of ammo are you using and have you lubed the buffer in the buttstock any?

The ammo used was PMP (Gazelle stuff) and Winchester (white box).  The buffer was a little dirty and was lubricated...but now that you mention it, I didn't clean it.

I had a similiar problem with my new Bushmaster.  I sent the upper to Bushmaster to be fixed.  To fix it, it appears that Bushmaster just placed red Loctite in the hole between the gas tube and the front tower.

However, I would be afraid to try this myself.  I would worry that the loctite would run between the crevice, and plug the actual gas port machined into the rifling.

Thanks.  It is a Bushmaster.. that he bought used.  I'm gonna play with it a little bit more. If I can't make it run, I'll suggest he send it to BM for repair.