Posted: Thu Feb 04, 2010 4:18 am Post subject: BC Variatons for JSB Exact Pellets

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....

Bob _________________ Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! Current Airsonal: .177 Diana 34, 1750 CO2/DPH, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP Carbine w/RIM (Grouse Gun), 2260 PCP Rifle (50 FPE).

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....

Bob _________________ Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! Current Airsonal: .177 Diana 34, 1750 CO2/DPH, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP Carbine w/RIM (Grouse Gun), 2260 PCP Rifle (50 FPE).

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....

Bob _________________ Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! Current Airsonal: .177 Diana 34, 1750 CO2/DPH, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP Carbine w/RIM (Grouse Gun), 2260 PCP Rifle (50 FPE).

it seems that besides the 25 grain pellet, there is an obvious sharp incline, peak and slower decline in BC as velocity increases, have you considered that the 25 grain pellet hasnt reached peak BC yet?

I don't have enough data points on the 25 gr. pellet.... I'm assuming that it is near it's peak BC on the graph and is about to start the decline just as with all the others.... The most powerful gun I have will only reach 914 fps with that pellet.... The best fit I could make through the data points was a straight line....

Remember, this data was plotted from a limited number of data points and it was complied by measuring the velocities at 1 and 26 yards.... By the time the pellet has travelled just 10 yards (if it started in the transonic range) it has already seen a huge decrease in velocity because of the low BC in the transonic region.... The only way to get better data is to use more data points and to measure the velocity at say 1 and 11 yards and calculate the BC from that.... If you had two Chronys, an even better method would be to shoot through both of them at the same time and have them only about 5 yards apart.... The problem is that over very short distances, even 1 or 2 fps will change the BC by quite a bit.... so ideally you need to caclulate the BC for each shot using two velocities and a known distance apart....

Like I said in the first post.... don't try and use the graph to get the exact BC.... it's only a best fit through a few data points.... It's just to show the trends.... and the effect of transonic velocities (ie over Mach 0.8 = 900 fps) on the BC....

Bob _________________ Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since! Current Airsonal: .177 Diana 34, 1750 CO2/DPH, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP Carbine w/RIM (Grouse Gun), 2260 PCP Rifle (50 FPE).

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