I have a Midwest Industries M-Lok rail with a Criterion Hbar barrel, and an aero low-profile gas block. The bottom of the gas block is pretty close to the inside of the rail. I don't have feeler gauges, but I measured it with a 13pt piece of cardstock, and two of those fit so it is at least .026 inches of clearance. I can squeeze the barrel and handguard together, but it takes a fair amount of force, as the barrel and handguard are both pretty stiff.
I do not see any signs of stringing or accuracy issues. I shoot with a sling a lot but I am not planning on getting a bipod or anything. Everything is all aligned and screwed and pinned together at this point, so I would rather not have to take it apart and find an ultra-low profile gas block for it. Should I worry or just drive on?