Originally Posted by oneofthose
Is there a general rule as to which powders require a magnum primer, and which do not? Is there an imaginary line somewhere on the burn rate chart that when crossed might require a magnum primer?
Not clearly defined by any stretch. In low end loads, it probably won't make a bit of difference. If I'm using a slow burning powder and there's a big powder column (like .300 win mag), it's a pretty good idea, particularly if it's going to be used in cold weather / hunting season.
I think with 2400 being a true double base ball powder, it doesn't have any issues getting complete combustion with standard primers. Some report it gets hinky with mag primers... just hasn't been my experience with the few times I've used that combo.
Some pistol powders like H110, 296, 4227 you just won't find any data that doesn't use mag primers.