Quote Originally Posted by ray1970 View Post
Doesn't the UBR use a standard carbine buffer/spring setup?

I know I've seen AR-10s with standard collapsible stocks.

Can't you just use whatever they put in those?
Longer bolt (ar10) inside a standard buffer tube requires a shorter buffer. Other option with normal stocks is to use an ar10 carbine tube, but a UBR is only available with an ar15 length tube.