Different years, different safety factors.

[Flintshooter]

Just started loading .38 Special again after several decades away from it.
Here we see pages from different years of a Hornady manual for 125 gr jacketed bullets.
The page on the left is a photocopy in One Book/One Caliber from what I guess is a recent Hornady manual. The page on the right is from an 80s something Hornady manual I kept from my first time around in reloading. Note the differences in starting and max charges for Unique.
I have learned that Unique went through a slight change a few years ago making it a little hotter. But I wonder how many there are out there like me that blissfully have loaded from old books unaware (as I was up to a few hours ago) that the game has changed slightly.

 

[buck460XVR]

If you look at the velocities produced by the similar charges, you'll see they are very similar. Don't think it's the Unique formulation that has changed, but the advancement of pressure testing. Probably why the newer book shows no loads developing over 1050fps(and only two at that), while the older book shows that as only a midrange velocity.

 

[rabid wombat]

Notice in the same two publications the change in Bullseye loading....

 

[jmorris]

Different people, environment, firearm, lot of components.

I have books with different data for the same powders, in the same chart (hp38/w231, H110/w296).

 

[Encoreman]

Flint, I feel your pain. I think if you had a copy of every mfg. book they might all be different. It is very very easy to make a mistake. Some older books give load data on powder that newer books don't. Double, triple check and start low and chronagraph and look at your fired brass they will "talk" to you if you let them. Mac