Ar pistol build

[LouBoyd]

There's no law on the length a pistol gas tube must be.. They aren't mil-spec. You can:
Ask your barrel maker to specify a compatible gas tube manufacturer,
Buy a gas tube and send it to the barrel manufacturer to determine the port location.
Cut a longer gas tube to fit.
Wrap a longer gas tube around the barrel to come out the right length.

The last offers the advantage of delaying the bolt opening a little.