Cases end up short and you’ll often find a doughnut at the bottom of the neck where you pulled brass which used to be shoulder into the neck, so you’ll likely end up neck turning. (Which is a good idea anyway, since your case neck ends up thicker as you smash the material of a .306” ID neck into a .241” ID form). What you’re wanting to do can be and has been done, and a little extra elbow grease will get you there. But it’s still a major pain in the butt which I honestly don’t believe is worth pursuing, even in the midst of this buying panic.

I’d personally be patient and buy .308win brass when you can find it, or try to find someone to trade for 308 win brass.

I couldn’t be bothered to waste time fireforming necks only to open up 243win to 308win. We fireform to move case bodies and move shoulders, which are case forming actions which simply can’t be done any other way. Simple neck up jobs like this should never be done by fireforming.