Some of the caliber and bullet design discussions have pointed me towards this direction for trying to gain a better understanding of how particular projectiles behave with regard to terminal effectiveness after having flown for any considerable distance; basically I found myself trying to isolate what effects resulting from internal and external ballistics on the bullets themselves can be replicated in order to evaluate, or at least produce a viable analog for evaluation in gel or other targets, and what other considerations are required for that. The overall goal for trying to understand this, for me personally at least, is to see at what particular distances (or at least approximate distance envelopes based on particular muzzle velocity and barrel twist) individual bullets still perform as desired terminally, without having to actually set up gel blocks at distance and try to hit them precisely, and instead be able to set up variable analogous setups for more controllable laboratory testing and better understand that ragged edge where the velocity, RPM, yaw, and design of the bullet conspire to no longer produce specific desired results on target. To better do this, my effort would be to very closely approximate a bullet having flown those distances, but without the logistical nightmare of trying to trying to compare multiple bullet weights, constructions, calibers at all kinds of incremental distances, or at least be able to answer rather specific question (e.g. comparing Mk318 and Mk319 performance at 200, 300, 400, 500m; comparing Mk262 and M118LR at 400, 500, 600m; or sorting out which of Mk318, M855, M855A1, Mk262, and 70grTTSX/50grTTSX/62grSP really makes sense at various distances).
A lot of data comes from hunting, but even amidst those data I quite often hear of instances where the shooter will assume they missed after a particular shot at range, only to find a blood trail from a wounded but still ambulatory game animal and no clear picture of why that bullet was unable to humanely drop the animal, while an identical load/rifle pairing was effective in very similar circumstances. I figure much of what is attributed to luck, or assumed to be fortunate shot placement (or bad luck/unfortunate shot placement) may still be entirely explicable by finding odd nodes of stability where some projectiles will tend to penetrate unperturbed. So…
Knowing the effective drag coefficients (of the bullet having been slightly altered with rifling along the flat) and being able to back-calculate velocities at particular distances seems straightforward enough, but this doesn’t seem to be a complete answer to that equation, but I’m not sure I have all of the other considerations in my short list, or if some of these are going to be so inconsequential that they’re not worth looking into further.
First one would be the attitude of flight: the particular angle of attack and yaw the bullet would experience at those ranges; I heavily suspect that the precise orientation of bullets with regard to their trajectory would affect tendency to deform/fragment/disrupt and how quickly that happens upon impact, especially in the first few inches of soft tissue. That said, I’m not exactly sure how to measure or replicate this all that precisely when we’re talking about somewhat extended ranges, factoring in parabolic trajectory into drag-induced orientations, precessional drift due to gyroscopic effects, and variable drag loads along ogives or at meplats/open tips in match/SOST type rounds: even with some extensive computational finite element analysis this might be a bit challenging to crack, but it should be possible to estimate and at least approximate this.
Second would be the rotational speed of the bullet at these ranges - I figure virtually every rifle bullet is going to lose speed in the direction of travel faster than rotational speed, so it should be possible to test at shorter range the same bullets from a tighter twisting barrel and get pretty comparable rotational speed, but it’s back to being yet another factor to include in the equation, since inherent gyroscopic stability is dependent on the specific RPM of the bullet at that moment, which I figure must in turn must affect terminal ballistics.
Some other smaller ones might be precise bullet mass effects (e.g. core-lokt or other exposed lead bullets probably change total mass as well as mass distribution), how the gyrostability and attitude of bullets are affected by rifling grooves being kinetically stamped into the flats on bullets, or even small stuff like temperature of the jackets possible affecting ductility in critical deformation points.
Of course, without some outside expertise I’m unlikely to get too far with this just myself beyond detailed thought experiments, but maybe the likes of Molon, DocGKR, and others could enlighten on this, or even push this farther towards meaningful hard data.
What stuff have I missed? Is this a bigger project that even I realize? Can a lot of this be readily sorted out by running the same projectile at a lower velocity through maybe a faster twist barrel and just shooting at 100m, or would those exact values be entirely too cartridge specific to provide any meaningful cost/logistics savings to achieve an apples to apples comparison?