Originally posted by Photocop
While I don't have any exact data, I have asked Mike McNett about the increase in velocity that could be expected with a 6" bbl. He said that for most of his loads the increase is about 100 fps and there is one load(I can't remember which one and I've since deleted the e-mail), that I think hops up 125-150 fps.
I know this is far from specific, but it's good for ballpark figuring.
I think you're spot on. The 200gr XTP's gain a little under 100 fps and the 135gr gain 100-150 fps as I recall.