Hornaday 450/400 3" ammo is loaded with .410" bullets to 2050fps. Seems this is the formula that was safe for all 450/400 3" rifles including rifles with bores at .411" and those at .408". Or so it says on the web site. See the Hornaday site for more information.

http://www.hornady.com/story.php?s=483

JPK