Here's some sound advice from someone in the know, got this from the 6mmBr.com site.
Jerry Tierney, a past NBRSA 1000-yard champion and .284 Win “guru”, has observed that overly tight chamber neck tolerances can cause accuracy problems. Jerry says “some guys who were running minimal neck clearance .284 chambers weren’t getting the accuracy they expected. If you open up the neck to allow more clearance, say .0015-.002″ per side, that seems to solve the problems. I can’t tell you exactly why–maybe it allows a ‘cleaner’ bullet release–but easing the neck clearance has helped many .284 shooters get better results.”
Found it under the 7mm Cartridge guide:
7mm Cartridge Guide within AccurateShooter.com