Is it OK for there to be a bit of dirty residue on the gas tube near the gas port? I just cleaned my AR rifle and found a slight build-up at the gas tube connection at the gas block. Also, unrelated, how many rounds in a certian period of time is damaging to the rifle barrel? Firing 20 rounds in a two minute period really heats things up,  but does it hurt anything? What about a forty round mag in a two minute period? Just wondering what you more experienced shooters thought.

Thanks

What make rifle and barrel do you have?

If it's chrome lined, you can fire way faster than that wiithout hurting anything.  A Colt 6920 uses the same barrel as the military M4, and they fire those full auto without problems.  With a stainless steel barrel I would more hesitant to dump mags through.

Regular 20" C/M barrel, 1 in 9 twist. Thanks for the info.