[Unclenick]

Alan Jones said primers change more often than people realize and the change isn't typically publicized. As long as the pressure differences the changes produce are within about 4% of what the previous recipe did (SAAMI's pressure standard deviation assumption), I expect it isn't considered necessary to say anything. 4% is about the maximum difference I've seen even going between magnum and mild primers has made. But that's not to say it won't affect your groups.

This is one reason to keep a chronograph. As long as they don't get old enough to start corrosion welding bullets to cases, if you keep aside a few of your last batch of loads, you can chronograph them on the same day with your next batch by alternating one old/one new until they're both shot up. This ensures barrel temperature rise and changes in the light falling on the chronograph and fouling accumulation all get averaged out. You compare the average of the old ammo velocities with the average of the new, then you can make a fairly easy charge adjustment to correct for a new powder lot, and new primer lot or a different primer or different brass or whatever. If you've kept past records of loads as you've developed them you will be able to look up how many feet per second change per grain of powder you get with that bullet.