Author Topic: Airgun Ballistic Coefficient (Read 515 times)

steveoh · « **on:** February 03, 2018, 03:25:40 PM »

Messing around with scope, I looked for ballistic coefficient numbers for different
Pellets and found this very useful page.

http://hardairmagazine.com/ballistic-coefficients/

Alan · « **Reply #1 on:** February 03, 2018, 03:44:16 PM »

Someone mentioned this data before. They're very close, but measured with a Labradar, and corrected for altitude, they do very some in the third digit. For example, the 24.4 grain AirArms I use, are .036, but measure .038 after correction. That's fair insignificant, unless the ranges are really long. Longer than I normally shoot anyway.

By the way, the X-Sight II is a bit tricky to set the digits, as you have to arrow over to get to the last digit. Good thing is is a set and forget. Just make sure the distance to target (shown in the upper left of the display) is set at the same as the sight in distance (say 50 yards). If it isn't, sighting in is messy! For some reason, that part of the ballistic program is never set completely off.

msurf · « **Reply #2 on:** February 24, 2018, 02:28:27 PM »

Thanks Steveoh,
I checked this out before as well while looking for B/C for the 244g Predator XXL .45. The closest I could come up with was for a round ball. I noticed you have a 909 as well..would you happen to know the B/C of this round?
Thanks,
Mike

steveoh · « **Reply #3 on:** February 24, 2018, 03:33:54 PM »

Quote from: msurf on February 24, 2018, 02:28:27 PM

Thanks Steveoh,
I checked this out before as well while looking for B/C for the 244g Predator XXL .45. The closest I could come up with was for a round ball. I noticed you have a 909 as well..would you happen to know the B/C of this round?
Thanks,
Mike

i found this:

Round ball coefficients are based on a velocity of 1000 - 1300 f.p.s at the muzzle.

.308 - .038
.311 - .039
.319 - .040
.330 - .042
.350 - .044
.360 - .045
.375 - .047
.380 - .048
.390 - .049
.395 - .050
.433 - .055
.440 - .056
.445 - .056
.451 - .057
.454 - .057
.457 - .058
.490 - .061
.495 - .062
.498 - .064
.500 - .061
.527 - .067
.530 - .067
.535 - .068
.562 - .071
.575 - .072
.600 - .076
.690 - .087

https://www.titanreloading.com/ballistics-questions/ballistic-coefficients-on-molds

rsterne · « **Reply #4 on:** February 24, 2018, 06:04:47 PM »

If you notice, the BC for roundball is proportional to the caliber.... just as it should be.... because the SD is proportional to caliber because of the square/cube rule....

Bb

msurf · « **Reply #5 on:** February 24, 2018, 07:37:09 PM »

Thanks Steveoh. Bob, I may be wrong, but believe I read on one of your posts elsewhere that the diabolo style pellet B/C is similar to a roundball, correct?

rsterne · « **Reply #6 on:** February 24, 2018, 07:59:52 PM »

Yes, a round-nose diabolo pellet, like the JSB Exact series, has a similar BC to a roundball of the same caliber and weight.... If the weight is lighter or heavier than a roundball in that same caliber, you can use the ratio of the weights to get a rough BC.... This is because a sphere has a Form Factor of about 1.55, and since BC = SD / FF, you can get a pretty good estimate of the BC of a roundball or pellet by using 2/3rd of the SD....

The SD is the weight in lbs. divided by the caliber squared.... so SD = Wt. (gr.) / 7000 / cal^2.... If you calculate the SD for your pellet, and then take 2/3rds of that, you will have a close estimate of the BC if it's a round-nose design.... Note that a "domed" pellet, like the RWS SuperDome has a lower BC, because the nose is not hemispherical.... and of course pointed, hollowpoint or wadcutters have a lower BC as well, for the same weight....

Bob