I took my rail off last night while I was cleaning my rifle and found this mess. This is probably 800ish rounds worth of fouling. The gas block looks almost as bad. Does this seem like a lot?

The rifle runs fine with all brass cased ammo I've tried but won't reliably feed steel case, so I'm starting to wonder if the gas system is leaking more than normal and the rifle acts like it's undergassed. I typically shoot PMC bronze or American Eagle.

How do I fix it if that's the case?

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Blow back at the receiver end is no issue.  The tube is inserted into the gas key on the bolt as a loose fit.  The gas either dumps in the receiver itself or escapes other places as the bolt travels back.

If you had a big leak at the gas block side I'd be somewhat more concerned.  Not too much regardless since it's a pretty high impulse system.

Don't worry bout it