243AI Neck Turning Help!

[Rookie]

My new rifle is chambered for 243AI with a .270 neck, I have a 243win that also has a .270 neck. When I load for the 243win I turn the necks to .268 before the first firing (Lapua brass).
Should I turn the necks of the 243AI before fire-forming with a bullet or fire form with pistol powder CoW & Poly fill and squeeze neck down to .268 so it'll fit the chamber? (Lapua brass) Dies are Redding bushing Comp. Full and neck. Redding Comp. seater.
Help!

 

[alf]

Either way gets you there.

Sometimes I prefer to blow things out straight before turning. If you use the COW method, you only have to size the necks to fit in the chamber, no need to take them to .268".