in fooling around a bunch loading different 50 bullets and powders it really comes to ones attention that the lots of powder vary quite a bit when using surplus powders. We have finally settled on using a chrono to work up a load. Knowing what velocity the best working load was at, we adjust the powder weight until the same velocity is reached. It's working out quite well with accuracy and functioning remaining fairly consistent. The powder however is varying up to 15 grains difference per lot.

Welcome to the wonderful world of reloading. You will also find that using the same powder and quantity in different brass makes will also cause variations.

Butch,
This is one thing that you need to be very careful of with .50 "surplus" powders... They are NOT "cannister grade" meaning that from lot to lot, their burn rates can vary dramatically... Always err on the side of caution when changing lots, and if you really like a specific powder, buy a BUNCH of it IF you can be assured of getting the same "lot"...
The military guys always load to a velocity "spec" so the weight of the charge is irrelivant on a "mil-spec" load...