There are many outer barrel sizes, Laylax makes: 450mm, 575mm, and 700mm outers for the VSR series. Your math should be right, the VSR-10 G-spec is about 330mm (IIRC) and the barrels for it are 303mm.
The difference in length comes from the fact that (obviously) the inner doesn't come out as far as the outer, just look into the barrel of your G-spec, the difference comes from the silencer treading. If you do get a longer barrel make sure you pick up some (2-4) barrel spacers to stablize the inner if your using a 150%
Oh, and I got a new dremel and swapped in the O-ring, my VSR-10 isn't losing compression anymore

YAY, Still need that stock piston Tsuru?
Has anyone else noticed the 200% VSR-10 springs that ACA has in stock for 20 bucks?