Thread Length:

Most AR15 and AR10 barrels have a thread length of 0.625". Many AR9 barrels have a thread length of 0.400". Some barrels may be longer or shorter, and you should measure your barrel to get the most accurate results.

Barrel Length:

Not all barrels are the exact listed length. For example, some 10.3" barrels are actually 10.25" long.

Handguard Length:

Many handguards are not the exact listed length. Aero in particular is really bad about this.

Listed vs actual lengths of Aero handguards:
7.3" → 7.25"
9.3" → 9.22"
10.3" → 10.25"
12.7" → 12.63"
15" → 14.94"

Shims:

A thick shim or Accuwasher can be used to move the muzzle device and adapter away from the handguard and gain more clearance.