I'm nerdy enough to enjoy this kind of thing, so I thought I'd pull some data for the discussion.
Experimental design:
'Efficiency' in this context means converting chemical energy in the propellant into kinetic energy in the bullet as efficiently as possible. More bullet energy, less powder, less recoil.
One way to assess this is to look at the powder in various cartridges, and see how many ftlb of bullet energy each grain of powder yields for each.
This analysis could be translated across multiple powder types by getting specific energy for each powder type, converting it into KJ of chemical energy per load, then looking at KJ of 'useful' bullet energy for each
However, that's a lot of work, so I chose to simply pick one powder that works across lots of cartridges to eliminate all that data collation and math.
I chose VARGET, because it's a common powder that can be used in a lot of different cartridges. For the purpose of this study I'm 'assuming' that this is a highly suitable powder that yields equally good efficiency across all these applications. That's probably not a fair assumption, but this isn't a particularly in-depth look at this topic, so meh.
I chose book max values from the hornady manual, and selected a 'normal' sort of bullet weight for each. Obviously all cartridges selected needed to have a VARGET load, as did the specific bullet weight selected. This does limit the study a little in that anything extremely overbore, or underbore does not have a data point. So no 338lap, 50bmg, no 45 colt, sadly no NE cartridges, etc.
Data:
Findings.
As a general rule, efficiency is improved as the bullet gets heavier.
There's a weaker correlation between velocity and efficiency (see 300savage vs 300WM as an example). This was surprising to me, as it goes against my initial assumption that 'trying to push a bullet faster and faster will always cost you efficiency'. It appears that just ain't so.
It's also interesting that actual efficiency is pretty similar in almost all cartridges this study looks at, with the possible exception of the 6.5CM, the 204Ruger, and maybe the 375Ruger.
I'd argue that even these outliers might just be artefacts stemming from the fact these cartridges 'actually' are either really well suited to VARGET, or a really poorly matched cartridge for VARGET, as opposed to being a pure case design factor.
Spitballing on some possible causes for the trends seens, I'm 'assuming' that there's some inherent characteristics of bigger, heavier bullets and the cases that use them that lend themselves to efficiency.
This might have something to do with one or more of the following:
A bigger cartridge means less energy lost heating up the case and barrel as there's more 'space' between much of the hot gas and the cold barrel surfaces.
More efficient acceleration of the bullet due to a larger surface area of the base of the bullet as a proportion of total internal cartridge surface area.
A larger overall case may have proportionally less tensile strength, potentially leading to less energy expended on expanding the brass to fit the chamber, and also a better gas seal around the case.
A larger initial volume for expansion in bigger cases helps with complete combustion of powders and improves initial temperatures, leading to more efficient bullet acceleration
A heavier bullet has more inertia which keeps the hot gases in a smaller area for just a bit longer, again leading to slightly more efficient combustion
Maybe something going on with the larger volume of the area 'behind' the bullet for expansion of gases.
The weaker trends around simple velocity vs efficiency make me think that it's not 'simply' that heavy bullets go slower, and therefore there's less frictional losses and less efficient acceleration as the bullet gets 'just a bit closer' to the maximum expansion rate of the expanding gases.
It'd be interesting to look at some heavy bullet, high velocity cartridges, see if this trend holds out. 50BMG as an example. It'd also be interesting to look at some really slow, heavy bullet stuff like 45Colt, see if velocity or pressure do actually have much effect.
It'd also be interesting to explore 'the same case, with different bullet weights', see if it's more to do with cross sectional area of the bullet and internal surface area of the case, or pure mass being accelerated.
I think digging into more detail across powders might prove that really, it's more to do with matching the 'correct' powder to the 'correct' case design than it is the intrinsic design itself. That too would be an interesting follow up.
For the purposes of 'general' trends towards efficiency, I think you'd want to look at cartridges offering the following:
- A big heavy bullet
- Not massively overbore, but also not massively low pressure.
- Probably a bullet on the higher end of weights for the cartridge that offers 'reasonable' but not crazy velocities in the 2500fps range.
