One other thing to remember...and I'd think it would be quite important. Bullet seating depth also causes increases/decreases in pressure. A bullet seated deeper in the case causes an increase in overall pressure...not necessarily a good idea in nice old rifles.

Consequently, .500's recommendation about neck tension is important. If you fire one barrel that might cause the bullet, for some strange reason, to seat deeper into the case, the result is a potentially significant increase in pressure in the second shot.

By the way, does anyone have the factory OAL for the .450/.400 3-1/4" NE?