When you look in a load manual for a particular cartridge, for a particular powder, for a particular bullet weight and shape, you’ll notice there’s a recommended starting load (powder charge weight) and a recommended maximum load. Even if you know you’ll be wanting, eventually, a maximum load for whatever reason, it’s always good to start with a load significantly lower than that and then work your way up, rather than starting with that powder charge right off the bat.
Gunpowder is produced in lots. This will be referenced by a lot number on the powder canister. Though gunpowder companies do their absolute best to ensure that every lot is exactly the same as every other lot, the unavoidable reality is they’re not. Variations do exist between lots of powder. That maximum load you built-up with a particular lot of a particular type of powder, that’s safe in your gun, might not be safe when using the exact same charge weight, out of a canister with the exact same powder name on it, but from a different lot.
For this reason, if loading to maximum, pay attention to the lot number on your powder canister. Try to buy as much as possible of that lot at a time. Minimizing variables is a GOOD thing. If loading with the “same” powder from a different lot, always drop the load 5-to-10 percent and work your way back up, looking out for pressure signs all along the way.
Also consider that some powders are temperature-sensitive, their burn rate and pressure characteristics can vary considerably given the surrounding air temp. The Internet is a wonderful resource for this information. For instance, in some shooting sports it’s necessary that ammunition used in it generate a particular power factor, a number derived based on the projectile’s weight and velocity. You can find commentary on which powders notoriously pick up or lose velocity in hot or cold temperatures.
Competitors in “action” pistol matches are mostly worried about losing velocity, so they can avoid not making power factor at the chronograph station at a major match, during which they test the ammunition for weight and velocity to ensure it makes power factor. “Flunking the chrono stage” means you shoot the match for no score.
Those loading to maximum should pay attention to comments on powders generating significantly more velocity, thus considerably more pressure, under certain circumstances. Or does the powder notoriously drop velocity in cold temperatures? That means it gains velocity, therefore pressure, in hot environments, therefore it’s not a good choice to use, at max loads, in hot temps if you worked up the load in cold temps.
For all these reasons, it’s a good policy to avoid maximum loads whenever possible. Having said that, if you simply must load to maximum, starting low and working you way up, paying attention to lot numbers, and knowing your powder’s temperature sensitivity can save you a world of trouble.
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