I take it you are handloading. I'd ascertain the LTL measurement and back off starting at 0.005 and work from there and you need to insure the pills are concentric in the neck. Even factory ammo is suspect in that area. Myself, I check every round with a Hornady runout gage or a Sinclair concentricity jig, but then, thats just me.
0.005 OTL will most likely cause you some bolt wipe and may or may not fit the magazine/floorplate feed, depending on rifle but it's a good starting point and yields enough jump so as not to cause an overpressure condition.
I never look at the first 'cold bore' shot as telling, why I shoot groups of 4. That first one is most always off in some respect, why, when I go hunting, I always pop one off (even at home prior to) and never touch the bore until afterwards.
I'd clean it well beforehand with BTE (or your preferred copper/carbon solvent) followed by good old reliable Hoppes using nylon bore brushes (I gave away all my bronze things long ago) and a nylon jag or patch puller from the receiver toward the muzzle. Never pull a brush backwards. especially a bronze one, Nylon brushes are more forgiving and can be pulled backwards but the patches need to be pushed with a nylon jag or patch puller (not pulling, though, pushing and do it with a CF cleaning rod, not a metal one.
Having said all that, your mileage may still vary but you eliminate many varibles.