I blind pinned a B.E. Meyers 249F on a 14.5" Cross Machine Tool M4 barrel (it originally was a 16" M4 barrel I had them cut to 14.5") that actually measured an over-all length of 14.625". I'm guessing the barrel manufacturer wanted to give an extra 1/8th inch leeway to guarantee the barrel would meet the 16" rifle barrel length minimum. The 249F measures an OAL of 2.75", and the included washer measured (0.125)
Using a dowel rod on a closed bolt (you can take the bcg's bolt and place it in the barrel's, barrel extension, just make sure the bolts in battery when taking the measurment;) place the dowel down the barrel from the muzzle to the bolt that's still in battery, then mark with a piece of tape where the dowel meets just outside the barrel and just past the barrel crown. See what the dowel measures to; this is your barrel OAL without a muzzle device.
Next measure what the length of the threads are that are used to make up your threaded muzzle (this measurement will be subtracted); measure the thickness of the flash hiders washer; and the OAL of the flash hider your using.
Start with the OAL of the barrel without a flash hider (+) the thickness of your washer (+) the OAL of your flash hider; add it all up and then subtract this measurement you end up with from measurement for the length of the threads that make-up your threaded muzzle. This will be the OAL length of your barrel with a permanently affixed muzzle device.
You can place the dowel down your barrel after installing the muzzle device; from muzzle to a bolt that's in battery; and the two measurements should equal out each other.