A few months ago I discovered that while working on another project.

I have a Armalite 6 position butt stock that the buffer/spring hole is 7 3/4 inches deep and has a square bottom.

I also have a Tapco 6 position butt stock that the buffer/spring hole is 6 13./16 inches deep at the edge. The bottom of the hole is angled from the drill bit. They didn't square it off.

All the external dimensions are the same.

That's 15/16 of an inch difference.