Disparity in Max Loads

[powerspc]

This got me thinking…

I was looking up some reloads for .357 Magnum rounds when something struck me as interesting. There were maximum loads listed from 10.4gr to a high of 18.8gr depending on the powder. That struck me as a huge difference (8.4 gr); if we lived in the Land of Oz and every ‘recipe’ listed in the manual shot identical, two things occurred to me;

• Is there any inherent advantage or disadvantage to ‘filling’ a case versus leaving room?
• Does anyone take this into consideration when choosing a powder, e.g., “I’ll start with the powder that has a max load listed of 10.4 because I will likely end using much less powder per round and save some money in the long run”

In the real world, I’m assuming that the determining factor would be performance; it just made me wonder though...

 

[AIAW]

Pistol cartridges are probably a bad example to use for this because the powders are typically on the very fast end of the spectrum and not always filled.

Typically you want a powder burn rate that fills the case as much as possible. For pistol, this is not always possible due to the correct burn rate needed - why it’s a bad example mainly. In bottleneck rifle cartridges, an undercharge versus total capacity can give a secondary detonation from uneven powder ignition and other negative effects. Undercharging rounds can be as detrimental as overpressure rounds.

There is an equilibrium between case pressure, chamber pressure, bore diameter, bullet weight, bullet start time, powder burn rate and charge weight (along with a few other factors - powder temp, etc). Ideally you’d get as much powder as you can safely stuff in there without it being over pressure to provide a consistent burn, thus consistent and predictable pressure.

This is difficult to explain across all types of ammunition though as there are just too many variables to give a simple “one size fits all” answer.

 

[chevy_man]

You have realized why the type of powder is important.


The most simplistic view is that you need a varying amount of powder based on its burn rate and the cartridge size to bore ratio.


Just follow the manuals, and if at all possible check with several before starting work-up. I've used data from a 80's era manual that was perfectly safe in my gun from that era, but was 10% or more over newer data with better measurement.



If you get into rifle powders you'll discover some powders don't hit max pressure, but max out the case volume. One of my favorite 25-06 loads is retumbo compressed well beyond what any manual will say, but it shoots great and doesn't flatten primers or leave ejector swipes on the brass.
Now, put that much h1000 in a case (slightly faster powder) and the pressure spike will likely cause brass failure.
Go way to far and stuff a pistol powder in there? You'll be picking up pieces of the gun more than likely.