RecoilRob,
If your mind is made up, I can't change it. I do have a "V" block, but I made it for 5 groove barrels, 3 groove takes a different included angle. If a .358" bullet will easily enter the neck of a case fired in the rifle, then .358" bullets can be used w/o problems. BTW, my small hole gauges are round on the end, I wasn't talking about telescoping gauges, which are used on larger holes.
Mike