I'm not a "seasoned reloader", although lately my wife has been sprinkling salt, pepper, and a little cumin on me.
Never-the-less, here are my thoughts...Reason 1
Everyone knows HP-38 and Win231 are the same, right? Well, yes and no. These powders are all blended. That is to say, if you bought the same powder in 6 months it might not be the same. That's why people who are really into shooting buy the 8 lb can. There's simply more consistency than buying eight 1 lb cans.
Think of it this way, imaging you're the Campbell Soup Co making something as ubiquitous as tomato soup. Sure you have a recipe and computer controlled cooking, but still last week's soup still might be more brilliant red and have slightly less salt. And too, Campbell is always bowing to "taste dejour" so salt content has probably been creeping downward for the last 15 years.
This is another reason why you can't go buy the much cheaper 1970 reloading manuals. The powder formulations have really
changed over the last 20 years. Conversely, if you were given
100 lbs of free HP-38 made in 1970, you'd need to buy an older manual to use it safely !!
Long story longer, if the powders were made 6 months apart they are probably not the exact same. Close, but not the same.Reason 2
When you're reloading the MAIN
thing to control is chamber pressure
so the gun doesn't blow up in your hand. Although lots of things contribute to chamber pressure, the 2 main culprits are the amount of powder
and the space it has to burn in
(in other words, the volume inside the cartridge case under the bullet). Now as any fool can plainly see, you cannot measure to the base of the bullet once it's seated. So we talk in terms of OAL which is a measurement we can control. Never the less, it doesn't tell us directly how much volume is under the bullet. Follow?
Along the same lines as "Under ObummerCare the price of medical care will go down." If you repeat a lie often enough, then some poor soul will actually believe it !! In the same vein, reloaders are really lying to ourselves and each other.
And every once in awhile, after throwing around the term "OAL" like it really means anything, one of us slips up and really begins to believe it's important !! What we really
want to know is: OAL minus the bullet length, as shown below....
If you understand this train of thought so far, then you can see that the length of the bullet, a dimension no one ever mentions, is critical to the proper understanding for the chamber pressure. So whenever I see "the same load" in different books that have wide variations then I know something else is going on, like 2 widely different bullet lengths.Closing
When you see how widely loads from different books can vary, you start to get an indication of why it's so important to begin loading at the "starting load" and work up. Starting 10% below the max load has been shown to fully account for the 2 big variations above and a host of other variables still unmentioned.
All the best.