Actually, I'm usually able to reproduce Weatherby factory ammo with handloads, because I have all of Norma's powders on hand.
Norma loads their ammo, so I break down a factory round and weigh the charge, then look at Norma's loading data and figure out which one matches the charge. Sometimes the match isn't exactly to the half-grain, but that's no doubt due to variation on powder lots. But so far, every time I've done this the velocity was very close to the factory ammo's.
However, some of Norma's powders aren't particularly temperature-resistant, so I often use another powder for my actual hunting loads.
