@centercut, the ultimate decision to break this into a new thread is outside my purview.......but I'm thinking we might need a new topic.
Ready for some theoretical and philosophical ramblings?
I've often thought (and stated online a few times) that the way we airgunners view and use BC is erroneously black and white. And I'm perhaps as much or more guilty than the rest of us. The general mindset seems to indicate that we view BC as much more than it really is. I've used and seen it used as a metric to convey % of retained energy, resistance to horizontal deflection (crosswind), as well as trajectory (how far the pellet does or doesn't "drop" at a given distance). And yeah, the BC is really about the best parameter that we have for summarizing all the influences a projectile is under as it gets from the end of our barrel to where it is going (external ballistics).
BUT, I've personally seen examples where the BC very obviously doesn't fully or even accurately describe what is going on with a pellet. Situations where measured BCs don't perform as well in the wind as a lower measured BC, and vice versa. Like the one you're implying: how/why does the .30 have a seemingly better track record at the more recent large airgun benchrest competitions, despite (or maybe in spite of) the low BCs people and companies seem to measure with it? And I say "more recent" but I believe Tim McMurray of MAC1 Airguns won one of the early EBRs (2013 maybe) with a .30 and a veritable monster of a USFT with an air bottle that looked like a toilet tank float.
So, I'm at work today and have been doing some online digging, with the intent of finding something that helps answer the question: "why does the BC not always seem to be as useful/accurate as we airgunners hope (and sometimes treat as such) it is?"
The easy stuff is that BC is dependent on weight, diameter, and form factor. The form factor is where all the GA vs G1 stuff comes into play. And those are really just drag profiles that are somewhat of a best guess. In other words, the BC is the sectional density divided by its form factor. And SD is just weight and caliber. So, our beloved ballistic coefficient is nothing more than how a particular caliber/weight projectile compares to the how the reference "best guess" form factor projectile would. If those form factors are a little off..........It's actually surprising to see how little is changed in ballistics apps by using GA or G1 for pellet or slug.
A conversation over on Gateway to Airguns and a typically useful Bob Stern comment about the coefficient of drag (Cd) lead me to fluid dynamics, and yes that applies to liquids AND gases. This little blurb from wikipedias section on external ballistics made me realize there are still a lot of unknowns in the field and how it applies to projectiles.
"Drag resistance:
Mathematical models, such as computational fluid dynamics, are used for calculating the effects of drag or air resistance; they are quite complex and
not yet completely reliable, but research is ongoing.
[7] The most reliable method, therefore, of establishing the necessary projectile aerodynamic properties to properly describe flight trajectories is by empirical measurement."
(In other words, best actually measured and if you're not measuring, realize that it's all theoretical)
So a long dive into the rabbit hole of fluid dynamics and I came across a new-to-me term that I think has some relevance: Reynolds number.
At its most simple the Reynolds number is the ratio of the inertial force to the viscous force and how that affects a projectile. The viscous force in our airgun shooting being the medium through which the pellet passe (the air) and the inertial force being how much inertia that traveling projectile is carrying. But it also takes into account the point at which laminar flow around the projectile becomes turbulent flow. At any given competition, hunting site, etc, the atmospheric conditions (air) that we're shooting through is pretty much what it is, no changing it. But the "Inertial force" can be tweaked through equipment choice. My simple mind thinks of heavy stuff going fast carrying more inertia. But that speed can only go so high before we start getting into the whole laminar versus turbulent air thing. There's also a thought that something with a bigger cross section creates more potentially destabilizing turbulence.
Lots of other internet sleuthing but here are a couple links that were informative in regards to the Reynolds number.
http://waterocket.explorer.free.fr/aerodynamics.htm https://www.grc.nasa.gov/www/k-12/airplane/socdrag.html https://www.grc.nasa.gov/www/BGH/reynolds.html This Reynolds number discussion even considers roughness of the projectile (how sharp are the rifling marks?). It takes into account the well-known phenomenon amongst us of accuracy falling to pieces above a certain fps. The Reynolds number doesn't (and won't) replace the BC but it was very interesting to me and I just wanted to share what I came across.
