GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: JungleShooter on June 27, 2022, 01:38:44 PM
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I just read in an airgun magazine the following line:
"Higher FPS gives higher BCs, even if all other factors remain the same."
I already understand that BC is actually not changing, but because our drag model is so out of line with most pellets we end up tweaking it by changing the BC at different velocities. So, that's not the issue.
It's whether higher velocity increases BC, as the article seems to state (and then again, my bean head might have gotten this all jumbled up...).
Any comments?
Thanks, ;)
Matthias
Source (next to last paragraph):
https://hardairmagazine.com/reviews/airgun-pellet-test-reviews/is-heavier-better-for-22-cal-slug-bcs/
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Since the article was referring to slugs, where the BC is calculated using the G1 model, the statement could be correct.... If it refered to pellets (or slugs), and the BC was calculated using the old "constant drag" model (which some websites still use! ), the reverse is true, BC's measured at high velocities will be lower.... As you state, if the BC changes with velocity, regardless of the reason, the problem is one of using the incorrect drag model.... If it matches the projectile, the BC would be constant....
As an additional comment about the chart in that article, where BC is plotted against weight, I see an interesting disparity between the measured results and theory.... If all the slugs had the same FF (form drag), then the BC would be exactly proportional to the weight.... If you look at the straight line plotted on that chart, and extrapolate it to the ends (16 gr. and 32 gr), you see that the BC for those slugs would be about 0.056 and 0.096, respectively.... Theory says the BC of a 16 gr. slug should be half the BC of a 32 gr. slug.... The lighter slugs would have been travelling a lot faster in the H.A.M. testing (because the use the same gun), so they end up with a higher calculated BC because of the mismatch between the shape of typical airgun slugs and the G1 drag profile....
British Ballistician, Miles Morris, has done calculations that indicate that the drag model best suited to typical airgun slugs has less drag than the G1 model in the low Transonic range (Mach 0.8-0.95).... You can read about his new drag model here.... https://hardairmagazine.com/ham-columns/some-new-airgun-slug-drag-models-by-miles-morris/ (https://hardairmagazine.com/ham-columns/some-new-airgun-slug-drag-models-by-miles-morris/)
A discussion of it on the GTA may be found here.... https://www.gatewaytoairguns.org/GTA/index.php?topic=187505.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=187505.0)
If Miles new drag model is correct (and more and more evidence is being collected confirming that trend), BC's of typical swaged airgun slugs measured between 900-1070 fps but calculated using the existing G1 model will be higher than they actually are.... I don't mean they are "wrong" at the speed they are measured, but that as the slug slows down, it's BC will not match up to the "advertised" number....
Bob
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They got it wrong in theory...as velocity goes up... drag goes up, B.C. goes down... to about 1500 fps...
I real life well sometimes projectiles wobble at a slower speed when the twist is barley sufficient... especially if lobbing them at longer range...
I look at some of these very short light rounds and my mind say's great for short and med distance... but once you are lobbing them way out there if over spun they will not follow the nose... in fact seems to me they would wallow nose up on the descending part of the trajectory...
I have not found much out there specializing in sub sonic ballistics...
it may sound counter intuitive... but when you add barrel harmonics and long boat tail slug the muzzle blast may make it wag it's tail more at a lower muzzle velocity than higher velocity...
so yep there is a lot to still be looked at for these subsonic specialist...