ORIGINAL: Roskoe
I have built a bunch of 6.5-284's with both standard necks and tight necks. The standard neck is .299 on my reamer -loaded rounds are about .005 smaller.My tight neck reamer runs .294 - and requires a littleoutside neck turning. Some folks I know are running necks as tight as .290 for thousand yard shooting.
Once again Roskoe has come through!
.290 is way to tight, you would have to turn way too much material off. If you don't want to turn your necks I wouldn't go any tighter than .294 and you may have to turn some then. I'm guessing here but I imagine your OD will be close to .294 when you seat a bullet in your case. If so, you will need a .296 minimum IMHO considering that you don't want to neck turn. Let me ask this, If you don't intend to neck turn why are you ordering a custom reamer, with a tight neck? The two are generally not mutually exclusive.
Making a Dummy round is simple. Just measure your bullet. then seat the bullet in the sized case with no primer or powder. Then use your caliper to determine where the base of the bullet is. (open caliper to the length of the bullet, then hold one side at the tip of bullet and look to see where the lower jaw is in relation to the case) You want the base to be approximately at the neck/ shoulder junction. You don't want the base up in the neck and you don't want it intruding into the powder capacity either. This round will be very helpfull for your smith to order a custom reamer. But, I say again, I'm not certain you need a custom reamer. Why not just ream with a standar .299 reamer. Is the approx .002 worth it to you? I don't think you are going to realize any benefit, especially considering the platform. Trust me a standard neck will do quite sufficiently IMHO