Just got this e-mail back from Silencerco, in case anyone is interested:
From: Chris Wilkening
The calibers listed for the Saker 762 are “5.7mm to .300 RUM”, would this include .300 Weatherby Magnum?
From: K.C. Hutchinson (Wed, May 13, 2015 at 8:37 AM)
To: Chris Wilkening
Cc: Silencerco Accessories and Merchandise <email@example.com>
Thank you for contacting us. You will be fine shooting 300 Weatherby Mag through Saker 762 so long as your barrel is 20″ or longer. Please let me know if there is anything else we can help you with.
Have a great day,