im thinking of slowly building a 6mm ppc benchrest. I'v read that theres alot of variations in neck size. I'd like to order a barrel & don't really know what neck size to go with. pretty much looking for the most common, accurate size. i read theres .262, .264, up to like .270. any reccomendations? i would be fire forming, as i read it would make better brass using the 220 russian.