This link

http://www.clymertool.com/cgi-bin/reamer.cgi?cmd=CMD_START&gunID=2&reamerID=7

will take to a drop down list of rimless calibres for Clymer reamer drawings. Both the 450 Rigby and 450 Dakota are listed.

From memory the 450 Rigby factory ballistics are 2300 with 480 grains and the 450 Dakota is 2350 with 500 grains.

In either case that is about 200 to 250 f/s less than they would do with normal full pressure reloads.

Mike