There's no hard and fast rule. You can simply work up a load ladder, and when accuracy turns to crap, and your groups open up WIDE, you've exceeded the bullet's abilities. That's what I've done with many different alloys in dozens of bullet styles in over a dozen guns. Most guns will shoot most alloys faster than most people think they should. Coated BN 16 alloy should be good for anything you can do in a 45-70, unless you are trying for 458 Win lite.
