A Seex Kreedmire will roll .620 BC's at 3050fps. Hint.
This ^^^^^^^^^^^^ and easily. No sense settling for less except that you already have.
I've had both the 6mm & 243, & will take the 6 over the 243.
But the flatbolt (old) Rugers had a lot of piss poor barrels, with some being just fine, so I'd shoot it a bit to see how it does before investing too much time in it.
MM