Been reloading since the 1960's. Never have done it and never will. The reason is that some companies that make powders give a warning every now and then that says something like this.
"This is not the same powder as we produced years ago but has the same name and number designation "
I understand that Unique of old has a different burning rate than the new and so does 2400.
Hi, X Ring.
When I first got down here 24 years ago, I started looking for local components to reload with when I got my gun permits. I didn't know any of the multitude of individuals who "bring stuff" from across the line and they would not have trusted me anyway. I got myself an old square can of Hercules 2400 made way-back-when. I still have a bit of it.
I can tell you for certain that 13.5 grains of 2400 from the square can -- which is probably a 1950's era can or maybe 1960's (someone will chime in that this post means nothing without photos but I have the can in my off-site loading room and could photograph it later if need-be) -- and 13.5 grains of 2400 from a modern Alliant container with a 170 grain 358429 bullet out of a .38 Special case gives the same velocity from either a 4-inch Heavy Duty or a 6-inch Model 28. Extraction is equal with both loads -- one-thumb extraction -- and although I have no pressure-testing equipment the primers look the same.
So if 2400 changed, it had to be waaaaaaaaaay back.
However: we have a large 8-pound container that is very old of Hercules Green Dot. This powder DOES produce a noticeable difference in loadings compared to newer Green Dot. Our "normal" Heavy Duty loading with Green Dot is a 160 grain Lee SWC and 7.2 grains of powder from a .38 Special case to give us about 1,180 - 1,220 fps from 6 inches with one-thumb extraction. The same charge using the "old" Green Dot gives us around 1,300 fps with sticky extraction and flattened primers. In one Model 27 (now 23) the cases must be almost pounded out. The owner of this revolver loads 6.8 grains of this particular lot of Green Dot to achieve about 1,180 with the Lee 160 SWC and normal one-thumb extraction.
Were the old powders a different mix? Or did some one accidently pour some old Bullseye into the 8-pound keg of years ago and then shake it up? I don't know, except that the owner of the 8-pound keg of Green Dot made so long ago we can only call it "really old" insists that it was still fully sealed when he got it. The Green Identifiers in the old can are a very pallid Green, hard to see and one has to look for them compared to the very obvious Green Identifiers in newer production. But again -- the owner insists that the can was sealed when he got it and I trust this person completely.
I can't tell you one way or another whether the powder formulas changed over the years. In my own case, if I'm mixing powders that are made new -- although perhaps from different lots -- I don't worry about it. If I come across something old laying around in some rancho out in the boonies I always assume it might not be what the container says it is and start loading as if it were Bullseye anyway. The exception, of course, is the Dot powders, which have the identifiers that make them so popular here. Powders come here (through a myriad of check-points that are looking precisely for reloading components) in a variety of ways that I will not go into but often are no longer in the factory containers upon arrival and the Dot powders are appreciated for this reason.
But in the case of our "old cannister" of Green Dot -- there is a difference we have noted although we cannot explain it.