More info on this gun, we did it  for testing 416 HSM  with a rebated rim,
to check in intermediate size actions, with the longer, stronger,  bolt lugs, Ed..  

Here is experimental action for 416 HSM in a heavy
 stock....We lengthened port a little, set bolt stop back ,
  opened bolt face fit the 416 HSM    rebated rim.  Ed.

Here is comparison of Montana PH to Pierce action we
modified to use for rebated rim 416 HSM testing..Ed.

416 & 460 HE AND 416 & 460 HSM .LONG RANGE AND HUNTING
Can be set up in *Pierce *X10 *ACTION for the 416 & 460 HE *
AND X20 action for the 416 & 460 HSM , Can get those
if guys need one...Ed

Here is picture of my 416 HSM test gun,... It is a big falling block we had
as a 8 bore years ago. Has a 34 inch , 1 5/8 "diameter, 1 to 10 twist barrel. .. Ed.