There's a fellow big bore shooter here that built his 500 Nitro 3" on a Ruger #1 with a fast twist 50 BMG barrel.
He was shooting 750 grain A-Max's off the bench into less than an inch at 100 yards. He actually shot his Silver Benchrest medal with it! (5 shots under an inch at 100 yards.)

He reckons he was only good for 2 groups per day, the nerves started to wander after that.

DC