rebarrel ?

Yes, any cartridge based on the 308 or 250 Savage will work fine fine fine.

Get the fastest twist you dare. 8tw minimum, 7.5tw better and 7tw seriously good for the 100grain plus copper bullets.

A 6mm/250 is starting to interest me though a 6mm/6.5x47Lapua is in there as well.

So many options, we could give more targeted advice if we knew what you want to shoot and what bullet you want to use.
 
"for the 100 grain plus copper bullets"

Does anybody make the 6mm heavier than 100 grains ? The heaviest I have found in .243 is Cutting Edge Bullets in 100 grain.
 
Berger makes a bunch over 100 grain. They range from 105-115 grains. Sierra makes some 107's. Hornady makes 105's, Lapua makes 105's. Nosler has 105 and 107's. One of these ought to do anything you could ever want to do with a 6mm heavy bullet.
 
Sorry if I wasn't clear but I meant COPPER bullets. I've shot plenty of 115's for years but we can't use lead here any more and the heaviest i've seen in copper is CEB's 100 grain.
 
7tw or maybe 7.5tw for 115s Jacketed and 100 copper or if someone makes copper even heavier.
 
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