Ballistic effects of case/powder volume?

[Keebler750]

It seems to be a repeated 'fact' that powder volumes near to 100% create the best internal ballistics, but I have never seen any really useful discussion of why this is, other than the powder column not being offset from the primer while horizontal.

Does the air space issue have other non-linear pressures affects within the chamber, and has anyone heard any useful science on this?

My speculation is that differences in air space, all else being equal, would effect the steepness of the pressure curve immediately after ignition, or how much lag time there was, and I further speculate that this would affect barrel harmonics due to the lag times with a 'mushier' ignition event.

What brought this thought process about was my prior experience with suspension systems and the compressibility of gasses and how that effects ride harshness.....

Uh, so.....any thoughts?

 

[canuckgunny]

My brain hurts just reading that. I don't have a PhD so....

Here's my thoughts:

How many cases in a load manual are 100%.... not that many.
Most loads are limited by pressure. Obviously you don't pick powders that allow a double charge to fit in a single case for safety reasons, but I've never heard of a powder being chosen simply cuz it fills the case 100% and thereby is 'more accurate'.

If a 100% case volume is good, is a compressed load better still? I don't know, but if you find the guy who can answer to that other stuff ask him.

I think powder characteristics more than the airspace affect the pressure curve. Airspace in and of itself likely has virtually nothing to do with ignition inconsistency but powder placement, or misplacement as the case may be could be the more likely culprit. If the powder 'sits' a little different in the case each time, the flame propagation from primer cup forward will be slightly different, I think this is the biggest argument for 'full cases'. The presence of 'air' so long as its volume is consistent is likely a non issue in and of itself.
There are many a pistol shooter who claims the first shot differs from those that follow because the recoil of the following shots 'settles' the powder in a consistent (rearward?) position that the first shot doesn't see.
On a side note, considering the massive complexity of shooting, I find it amazing that more 'science' minded types aren't involved in the sport. Good to have you.

 

[acrashb]

A 100% load can't shift in the case, so the powder presentation to the primer flame is the same each time, hence more consistent ignition.