I came across these online, and thought they'd be perfect for a couple fiberglass AR freefloat tubes I made. It just happened that I used a 1/2" bit for my vent holes so they go right in!
Impact Weapons Components
This will be perfect for hollow stocks too, such as my HS stock, it'll be getting one soon.
FYI When ordering, it asks you to select a handguard but I called and confirmed it doesn't make any difference which handguard you select. It came with 2 pairs of allen screws (long and short) to snug it on the inside and it uses a split ring, like a very thin keyring that fits into a groove on the outside of the stock. Due to the design it can't be made "flush" with the stock, but it will only protrude about 1/16" if you use the upper groove (it has 2 grooves to choose from).
It's limited rotation to boot
Oh and the best part, made in the USA
Impact Weapons Components

This will be perfect for hollow stocks too, such as my HS stock, it'll be getting one soon.
FYI When ordering, it asks you to select a handguard but I called and confirmed it doesn't make any difference which handguard you select. It came with 2 pairs of allen screws (long and short) to snug it on the inside and it uses a split ring, like a very thin keyring that fits into a groove on the outside of the stock. Due to the design it can't be made "flush" with the stock, but it will only protrude about 1/16" if you use the upper groove (it has 2 grooves to choose from).
It's limited rotation to boot
Oh and the best part, made in the USA