Just an FYI
Many lowpro gas blocks are designed to sit against the shoulder these days on the assumption that you are using a free-float rail. Always good idea to measure just to be sure if the manufacturer doesn't specify.
If you have a dimpled barrel and a set screw gas block it might be easier to check. The screw hole for the gas block SHOULD line up with the port, and the dimple in the barrel SHOULD be exactly opposite of the gas port. The port in the block should be big enough to function even with slight misalignment