Is this lawyer talk or a valid concern?
I think its mostly lawyer talk. Kind of rare for the fitting not be be proper.
Not all that rare... sometimes they come that way from the factory and need to be warranty repaired.
Barrels can be and are often swapped between receivers but mating perfectly to the new one without some fitting is fairly unusual... luck of the draw. Correcting a slight mismatch isn't all that hard to do though.
There's a wealth of info regarding most aspects of everything related to these arms HERE:
http://www.go2gbo.com/forums/index.php/topic,26264.0.html
And HERE:
http://www.go2gbo.com/forums/index.php/topic,116451.0.html