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The reason you dont mount the gas block flush with the barrel shoulder

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This comes up very often. Guys take there new gas block and butt it up to the shoulder of the barrel. This is incorrect unless stated differently in your gas block instructions.

The reason is the proper gas block location allows for an A2 handguard endcap to be placed between the gas block and barrel shoulder. The thickness of the hanguard end cap is typically about .025


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That is also about two business cards thick. I actually have two strips of business cards taped together in my tool box, just for that gap. If you're unsure, it's safe to use the gap in all installations.

The gas hole in the barrel is always smaller than the gas hole in the gasblock. It's like covering a gopher with a bucket. You have room for adjustment. If you rest a gasblock in the shoulder, you run the risk of smashing the end of the gas tube into the gas key on your BCG.

This is very important for piston systems. That little bit of distance can be the cause of a crushed piston rod.

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