The Website said:The stocks are offered in two sizes: M16A1 length and the 5/8-inch longer M16A2 length stock system.
I'm kind of slow. How does the tube size not matter?
They will work on any tube. Rifle or carbine. Milspec or commercial. The OD is large enough to accomidate any of them and when put on a carbine tube, it has a small clip to attaches in one of the adjustment holes. When going on a rifle tube it uses the screw on the butt plate just like a standard rifle buttstock.
I got mine a year ago and haven't looked back.
It's worth the money, especially with the $50 discount.