My target rifle is chambered for 6.5x55, but the throat is very long. When the 139grain Lapua Scenars are seated to an OAL of 3.125" they are 150thou' off the rifling. I can't seat them out to touch the rifling for fear of the bullets falling out!
I would like to rechamber to 6.5-284 or 6.5-06. If I go for 6.5-284 how much will I need to set the barrel back? Just a little over 150thou'?
The 6.5-06 is trickier because its head diameter is smaller than the 6.5x55's. How much should I set the barrel back? What about with the Ackley Improved version?
Any help would be appreciated.

Why not just set the barrel back 1 revolution and rechamber for the 6.5x55. You are already geared up for it and IMO it's a great cartridge. derf

Even with a 30" barrel and loads approaching maximum I'm lucky to get 2800fps. So I'd like to get maybe another 300fps as well as having the bullets touch the lands. Everything counts at 1000 yards. I don't mind getting more gear to reload - that's half the fun!

Personally, I would have the barrel set back one turn and rechambered to the 6.5x55 BJAI. This would, IMHO, solve your problems in the simplist and most cost-effective manner. Contact Precision Shooting Magazine and obtain the excellent article by Bob Jourdan on his journey to the Ackley Improved version of the 6.5x55. Phone number is 860-645-8776. You want the issues of March, 1992 and August, 1992. Good luck in whatever you decide.

Ol' John