Grant,
They have been made up to the 577 T-Rex to my knowledge, A-Square rifles used to make the odd one.
I have 3 of them, one in 416 Ruger, one in 450 Ackley Mag and the other in 500 Jeffery.
They are slightly longer than a Mauser 98, and are better suited to the longer magnums and such.

Regards,
David.