Rec ext threads...how long?

[KyAKGuy]

Does anyone know how long the threads on the receiver extension should be?
I as because i have noticed the various amount of threads that are showing when
the stock is in the fully closed position. Some times the stock mates rite up to
the castle nut, and sometimes there is 3/8 or so of the receiver extension
threads showing when in the shortest position.

Not that it's a big deal, but just curious.

KyAKGuy

[Robb Jensen]

It varies a little from the manufacturer of the receiver extensions.
All that really matters is that it's installed far enough into the lower that the buffer retainer is held correctly but not too far into the lower that the bolt carrier hits it when assembling the upper onto the lower. In the old days sometimes you would have to file the top of the receiver extension a little to get it so that the buffer retainer would be held well.