Whiskey, this topic came up quite a bit when the Vltor stocks came out. The carbine version will not work with anything but Colt tubes. Colt tubes aren't marked, I doubt if Colt makes them, most likely subcontracted to others like LMT or others.
I have the RRA, Colt and Bushmaster stocks and extension tubes, if you put RRA/Bushy stocks on the Colt tube they'll rattle, because the rail sections are wider on the stocks. If you try to put a Colt or Vltor stock on an RRA or Bushy tube, it will not go in at all.
Colt's is 4 position only. Unless RRA changed its tube dimensions, it's not milspec. I see ads all the time touting that but only Colt's is. Wes at [url]www.mstn.biz[/url] will be selling these tubes, I suggest you get it from him. I got a Colt tube from Brownells for $120, Wes' units are made by the same folks that make them for Colt and go for much less.
The top is RRA, bottom is Colt, complete with their respective tubes. RRA/Bushy sling loop swivels, Colt's is fixed. The section immediately ahead of the release lever chevron on the Colt is not curved but perpendicular to the stock. Colt's material is also better, the checkering on the butt pad is sharper. The Colt tube is better made as well, without the machine marks still left visible on the RRA/Bushy units.
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