The base formulas have not changed, according to Alliant, but the more complex aspects have changed - flash suppressant, stabilizer, etc. - so you can expect about the same variance as you would from lot-to-lot plus! changes from age. As nitroglycerin and nitrocellulose age, they lose potency. How much? Who knows? It's not completely clear. Hopefully,
@Slamfire can show up and provide some input. He's made a sincere study of smokeless powder aging and deterioration. It's scary how much more magic there is in smokeless than science.
Hopefully the OP is not using 1964 powder. Stuff that old could be good, could be deteriorated. And I absolutely agree that the secret herbs and spices could have changed since then.
I did talk with Accurate Arms about powders. They had a standard pressure curve for their powders, and they blended fast and slow stocks to plus or minus 5% of their reference pressure curve. At the time, Accurate claimed the industry standard was plus of minus 10%. Not certain if that was plus or minus 10% from the mean or not.
One day, I am going to go to a bourbon distillery and ask, not how they blend their bourbons today, but how do they make sure that their bourbons taste the same as the originals before Prohibition. Almost all distilleries now, the names they have, they purchased. Few distilleries survived Prohibition, so unless they hired the original master distillers, before the corporate taste memory was gone, then how do they know today's flavors are the same as what the founders bottled?
Booze aside, I did check the date of the 43th Lyman handbook. It is from 1964. My 41st Handbook,
is from the late 1950's. I only have a few manuals from back then, the rest I tossed, but my 41st handbook does not have pressure tested data. The loads were chronographed. Cheap chronographs did not appear until the 1990's, so having a velocity was great. But the pressures were estimated the old fashioned way: physical estimates of pressures. That is, primer flattening, sticking cases, etc. And that is a horribly inaccurate way of estimating pressure. My #7 Speer, the max loads in that book will positively cause blown primers in some of my firearms. There are some hot loads in there.
Those old manuals, what is in there for loading data, should be considered in the same light as old ads for health products,
or the asbestos smoking pipe:
Not only are you inhaling horrible tobacco smoke, but surely also asbestos fibers, oh Joy!
Do notice, the Scottsman and the Irishman are on the opposite side of the line. Putting them next to each other would surely have started a fight.
If you don't have a modern manual, with pressure tested data, go to Alliant's web site and start with the current data they give.
