no luck in build it yourself, so I thought I would try here
I have a Vltor modstock which the receiver tube is 1.15 " diameter with flat back and peaked threads which I assume is milspec.
I have a Magpul commercial stock on a 'commercial' tube with flatter peaks on the threads (it seems) and the slanted back, but the tube measure 1.15" as well. The tube also fits in the modstock. It also has alot of play in the CTR stock until you lock the friction lock. Anyone else measured their Magpul commercial receiver extensions? I have another Rock River with a Magpul commercial kit that I will measure tomorrow when I go to my shop. I might add that there are no problems with this, I am just curious.