If I remember correctly (and that is a big if) Varget will gain about 15 fps from 0 F to 100 F. So going from 40 to 80 should give you a gain of about 6 fps.
Changing brass is likely the cause of your drop in velocity. The vertical stringing with a "new" load is likely your barrel settling into fouling. At this point I would say to do the test again with a fouled barrel to see how it performs for you.
Machine guns are awesome until you have to carry one.