Re: Lee Collet Die question
I have used Collet dies on a couple of odd cartridges and wildcats, shortened (e.g. 7BR for 7x33 Sako, 260 Rem for 6,5BR), as is, or with a washer/washers to get the proper distance. Itīs the distance between shell holder and neck shoulder junction that has to be _slightly_ longer on the die than on the case. If itīs too short the case body will bulge and the cartridge might not/wont chamber. If itīs too long only a small part of the neck will be sized and grip the bullet. This might or wont be a problem, far less than too thin a washer.
According to CIP figures: The minimum washer thickness is .1 with the 243 Win, but would be only .016 with a collet for the 6mm Rem.
As the 243 Win collet is made for a cartridge with 20° shoulder, and the 284 has 35°, the washer _may_ have to be thicker than that to avoid contact out on the shoulder edges.
This might seem like too much, but when the quirks are straightened you have a neck sizer as good as any of the standard ones.