I don't think it will have much if any effect at all. If you are a handloader who has worked up loads with a bullet that has slight variances in diameter (in the ten-thousandths of an inch) and done more than fired one group and called it good, then your load has already taken in to account the variances over the average pressures in your string of shots. Bullet diameter is a funny thing. I have a Swedish Mauser target rifle with the characteristic over-sized groove diameter that shoots under MOA with .264" bullets. I have a No1, MKII .303 that has a .310" grove diameter and I shoot .314" heat treated cast bullets through that. My Benny Rifle 218 Bee has a .223" groove diameter and I'm supposed to shoot .223" bullets to avoid "pressure problems", but I shoot .224" bullets with absolutely no problems. Serious cast bullet shooters know the simple truth about rifle barrels. The average factory barrel is seldom nominal groove diameter.~Muir
PS: There is the possibility that severly undersized bullets at max pressure could cause erosion, but a ten thousandth of and inch will probably obturate at the usual pressures.