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rsterne · Moderator

As many of you know, I have done quite a bit of research on Ballistics Coefficients.... After I did my second round of pellet testing at a higher velocity than the non-PAL round I did originally.... I began to realize that BCs were not constant but varied with the velocity.... Wadcutters tended to get worse as the velocity increased while round nosed pellets tended to improve (over the two speeds I tested).... Hollow points, domes, and pointed pellets were mostly in between.... Since I now have a couple of PCPs where it is very easy to adjust the velocity.... and since JSB have recently introduced two new pellets in .22 cal (the 13.4 gr. Exact Jumbo RS and the 25.2 gr. Exact Jumbo Monster).... I decided to spend a day shooting all five .22 Exact series pellets over the Chrony at various power settings at two known distances.... and then calculate the BCs using Chairgun.... I started with the RVA cranked to the maximum, then used 3, 4, 5, and 6 turns out.... plus added some data from my non-PAL testing for a couple of the lower velocities....

This is NOT an exact science.... just a few fps variation in the velocity can change the BC by 10%.... I shot 5-shot groups over the Chrony at 1 yard and at 26 yards and used the average velocities to calculate the BC.... Then I plotted the data on graph paper for each pellet and tried my best to fit a curve to each set of data.... It wasn't easy, but this is the results:

The first thing that becomes obvious is that there is an optimum velocity for each pellet.... in terms of where the BC peaks.... Secondly, the heavier the pellet, the higher that optimum velocity is, and the higher the peak BC.... Thirdly, by the time you get into the transonic region, pellet performance suffers dramatically.... and these efficient, round nosed pellets are down to BCs typical of wadcutters, or at best, domed pellets.... Velocities in the region of 1050 fps yielded BCs of only 0.0130 -0.014.... I couldn't test in the supersonic region in consideration of my neighbours.... but one would expect the deterioration to continue....

Possibly the biggest revelation, at least to me, is that published BCs are only a rough guide.... Stating that "Pellet X" has a BC of "Y" means little unless you also specify a velocity range.... In fact, a pellet will actually have a different average BC depending not only on the muzzle velocity, but also on the distance to the target.... as it is constantly slowing down along the way.... The only true way to find out the trajectory of your gun.... or how much of it's muzzle velocity and energy it delivers at a given range.... is to test it....

Please don't try and get actual BC numbers from the above chart.... Its only purpose was to try and show the trends that I found in todays testing.... Rolling Eyes

Bob

Slavia · Moderator

Thanks for your efforts! I knew BC changes with velocity, but I didn't realize it was that much.

rsterne · Moderator

Using the information above, I decided to test some JSB Exact Series pellets at the maximum power lever I currently have available to see just how much the air resistance slows a pellet over the first 5 yards relative to it's velocity.... Here are the results:

It's pretty obvious that in the transonic region the BC takes a dump.... and show how important pellet selection is in a high powered gun.... Shocked

In case anyone cares.... I tested a Hobby and it was right at the speed of sound.... it lost ~100 fps in 5 yards (over 16% of it's energy).... and the initial BC was only 0.007.... Rolling Eyes

Bob