I bought an upper assembly a few months ago from Bison. I'm getting some black powder residue on the barrel in front of the gas block after each trip to the range, even after only a few rounds. So, there is gas escaping. Of course it wouldn't even be noticable if the barrel wasn't stainless or if the gas block was hidden. Gun function is fine and accuracy is acceptable and getting better. So I'm thinking this is not an issue, but figured I'd ask to be sure. It's an SPC barrel with low profile gas block.