Al in Mi wrote:most rails have a notch along the top for the gas tube to follow, so the top part of the gas block should fit thru it as well. Getting access to the adjustment screw might be a issue thou.
Look up Just Rails, they have a bunch of different rails at decent prices. Buddy has a few but all his builds been with .750 gas blocks.
Correct there is a space to allow the gas block to go through. Issue is even running a .936 journal and a non-adjustable low profile gas block, you're close to maxing that out. .875 with an adjustable is higher than that. Unless you can find what basically equates to a low profile gas block converted to an adjustable (small set screw drilled into it).
I looked at my barrels with a journal bigger than .750". A .875 journal with a low profile non-adjustable SLR gas block, a bcm handrail is maxed out because the bottom of the block, which there isn't much sticking down, is almost touching the rail. The 12.5" BHW 450 barrel has a .950 journal, running an SLR non-adjustable gas block and an ALG handguard (which is a little on the bigger side for a slim), has a little extra room, but not enough for an adjustable. Thought my aero quantum would have enough for a .875 adjustable gas block, doesn't look like it. I didn't think an SLR rail would have enough, but they specifically say they do.