I would do 6.5PRC in your boat if you can find the supplies.
A 144 in 6.5CM you only need 2650fps to make powder factory which should be possible with hand loads in a 20inch.
Used twice at the range brand new. Glass is pretty nice but I am upgrading to something with a reticle.
Updated with photos... arca plate not included.
$399 Shipped Paypal F&F . Price drop!