.25 cal. Ballistics Coefficients

[rsterne]

I spent the day measuring the Ballistics Coefficients of all the .25 cal. pellets I have.... I used my Hayabusa, set up for about 960 fps with JSB Kings.... I shot 5 shots groups inside with the Chrony at 1 yard from the muzzle.... I made sure the tank pressure stayed between 2000-3000 psi for all strings, which is 100 psi above the regulator setpoint of 1900.... I then repeated all the strings outside with the Chrony at 25 yards from the muzzle.... and I even shot all the strings at the same part of the tank fill as I did the first time to be as consistent as possible.... The BCs were then calculated using ChairGunPro.... Here are the results.... Remember that these BCs are only valid for this particular rifle at the velocities specified.... although they should be a good indicator of what to expect....



Here are a few comments.... The JSB Kings had the highest remaining Velocity at 25 yards, and the tightest Standard Deviation at that distance.... I was a bit disappointed in the BC of only 0.024, but then I thought about how fast they are being driven, and it's very possible (in fact probable) they are beyond the sweet spot of the BC curve at 960 fps.... The H&N Baracudas (aka Beeman Kodiaks) had the highest BC at this power level.... The maximum Energy went to the EunJin Pointed pellets, both at the muzzle and at 25 yards.... but of course they had the lowest velocity as well....

As far as I'm concerned, a .25 cal is all about power and range.... Based on that, I'm only going to test those pellets which had over the 36.2 average FPE remaining at 25 yards for accuracy.... That means only those eight pellets with a BC of over 0.018.... Those that show promising accuracy will then get more work to determine the velocity where the BC peaks.... I'll keep you posted on further testing....

Bob

[MSEO1]

Wow! That is great info, Bob. I have been having some difficulty finding BCs for 0.25 pellets. I think you covered all of the brands I currently have. BTW, how many hours are there in the day out there in Coalmont?

[rsterne]

I shot indoor in the morning and outside after lunch and entered the data before dinner.... not too bad for 17 different pellets including all the calculations....

Bob

[Dukemeister]

Very useful stuff, Bob. BC is obviously one of the key parameters we have to estimate to calculate trajectory, and it's quite elusive and of course it's a variable. One question I have is what units are associated with the BCs in your table. In the ChairGun database, and everywhere else I look, I see only a number and no units. To add further confusion, the BC shown in ChairGun doesn't change when you switch from imperial to metric units. Of course the conversions could be handled internally by the software and the ChairGun outputs are likely correct, but it still begs the question... What units? Since all the articles I have read (admittedly on-line, like Wikipedia) use imperial units ft, fps, fpe, in, gr, lb... etc. do they all imply BC are measured in lb/sqin?

[teddy]

Bob
I admire all you work you do with different pellets and air guns, and keep track of it all. You must have a big hard drive. Over time I have looked at your website. How you get time for it all? Would love to come to that part of B.C.


ted

[rsterne]

I never thought about the "units" for the BC.... I'm guessing it's a dimensionless number like most Coefficients are.... Things like Drag and Lift Coefficients in Aerodymnamics are the same in Imperial and Metric systems, so must be dimensionless.... The theoretical BC is the Sectional Density (SD) divided by a "shape factor".... For a sphere, that is supposed to be 1.55.... That is why larger calibers tend to have better BCs because the SD is often greater.... No substitute for shooting and measuring it, though....

Bob

[Dukemeister]

You may be right, I believe the answer is buried in this article in method 3, which is part history lesson and part science.
Although the summary paragraph does suggest BC still comes out with units... ib/sqin:

http://www.exteriorballistics.com/eb...ed/5th/221.cfm

So, the ballistic coefficient actually is a scale factor. The BC scales the standard drag deceleration of the standard bullet to fit a nonstandard bullet. However, the BC works in a reciprocal manner. That is, the higher the BC of a nonstandard bullet, the lower the drag is compared to the standard bullet. This is alright, because it means that the higher the BC of a bullet, the better will be its ballistic performance. The physical units of the BC are pounds per square inch (lb/in2). The BC value for the standard bullet then is 1.0 (weight 1.0 lb, diameter 1.0 inch, and form factor 1.0 by definition for the standard bullet). Ballistic coefficients of most sporting and target bullets have values less than 1.0, and generally BC values increase as caliber increases. A bullet can have a BC higher than 1.0. For example, some heavy 50 caliber bullets have BC values greater than 1.0.

There will be more reading on this matter, but work and income tax taxes (sic, auto correct: I thought I typed take) precedence... sigh. Back to it.

[rsterne]

I continued with the testing today.... What I did was back the power of the gun down so that the muzzle velocities were close to the 25 yard velocities in the first set of tests.... I am trying to approximate what happens between 25 and 50 yards.... The RWS Hollow Points were dropped as they were not very accurate so would be hopeless at 50 yards.... I ran out of the EunJin Pointed, so had to leave those out as well.... The testing followed the exact same procedure as yesterday, with the following results....



I am concentrating my efforts on the JSB Kings as they are obviously the most accurate out of this gun.... The plan is to continue backing down the power so that each chart moves out about 25 yards for those pellets.... There will be some gaps for other pellets, but the data should be continuous enough to show the trends for the BCs.... More to follow....

Bob

[rsterne]

Third set of data.... approximating BCs from 50-75 yards in my Hayabusa....



The RWS SuperDome isn't accurate enough to continue further testing in this gun.... More to come....

Bob

[SeanMP]

Wow ...That's disappointing that the RWS isn't giving you the accuracy....fantastic BC. It flies well.