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rsterne · Moderator

For several years now I have been telling people that if you drive pellets too fast the Ballistic Coefficient will decrease, and the wind drift will increase.... I recently downloaded the latest version of ChairGun Pro 4, and it has some quite sophisticated models that show how the drag coefficient varies against velocity, from subsonic up to over Mach 3.... These models all have one thing in common, and that is a drastic increase in drag in the transonic region, typically Mach 0.8-1.2.... They have models for airgun pellets, spheres, .22 rimfire, cylinders, flat nosed bullets, and the G1 ballistics model that most BCs are based on.... When you input a BC for a pellet/bullet and select a model, the new ChairGun uses the Drag Coefficient at each velocity as the projectile slows down to calculate the next velocity increment and the drop and wind drift.... If you select a different model, you get a different result.... Likewise, if you select a different velocity, you will be using a different portion of the drag curve, and the results will therefore reflect that....

I set up a file to use four different .22 cal pellets/bullets and after selecting the appropriate model and BC for each, I input various velocities and recorded the wind drift at 100 yards and graphed the results.... I used a 10 mph wind at 90 degress to the line of flight, and here are the results....

This is the base data I used:

18 gr.JSB Heavy, BC= 0.036, GA model (airgun)_
27 gr. .22 Short, BC = 0.084, RA4 model (rimfire)
41 gr. RWS RN (basically a 22LR bullet), BC = 0.12, RA4 model (rimfire)
41 gr. Bob's Boattail, BC = 0.12, Custom model based on the Cd curve from the Kolbe Drag Calculator

The Cd plot from the Kolbe Drag Calculator is in the same format as the models used by ChairGun, so I just copied the data into the Custom model.... If you look up the BCs of .22 rimfire bullets, you will find a horrendous variation, so I took an average, and ended up using a BC which equals the Sectional Density, therefore using a Form Factor of 1.00, and I did the same thing for my Boattail.... Note that the above data is highly theoretical in nature, especially for my bullet, and measurements have not been made to confirm any of it.... I have, however, studied the BC of JSB Exact Pellets, and proven a rapid increase in drag exactly as predicted....

The most interesting thing, IMO, is that all the different pellets/bullets exhibit a minimum wind drift in the high subsonic range.... It occurs just after the drag coeffient starts to increase (the "Drag Divergence Mach Number"), which makes sense because at first the extra bit of velocity makes up for the higher drag, and the bullet soon slows into it's efficient range.... If you look at the velocity where the least wind drift occurs, and even more importantly the range over which it is the lowest, you get the following:

18 gr. JSB: 11.76" @ 900 fps (800-1000)
27 gr. .22 Short: 5.01" @ 1000 fps (900-1070)
41 gr. .22 LR: 3.49" @ 1000 (900-1070)
41 gr. BBT: 1.88" @ 950 (800-1000)

The rapid increase in the drag as the BBT approaches Mach 1 is due to the 60% meplat, a feature not found on the others.... This flat nose drastically increases the release of energy on impact, reducing penetration while creating a bigger diameter wound cavity.... It also lends itself to adding a large HollowPoint cavity without changing the BC other than by the slight loss of weight.... The trade off is that above Mach 1, the flat nose is, literally, a drag.... Fortunately, in airguns, we don't care.... Also note that the wind drift should be less than a .22 rimfire bullet of the same weight right up to Mach 1....

I think this analysis is pretty exciting.... It shows clearly why you should stay below Mach 1 with any airgun pellet/bullet.... and in fact from a wind drift point of view, 900 - 1000 fps is the best place to be, depending on the projectile.... There isn't a lot to be gained by adding 50 fps in terms of flatter trajectory, and every gain there is accompanied by a loss in resistance to crosswinds.... Lighter pellets, or ones with a poorer shape and a lower BC.... will start losing ground at an even lower velocity.... Trajectory we can deal with using holdover and MilDots.... Doping the wind is a lot harder....

Bob

rsterne · Moderator

I now have some new information.... I have quite a bit of data on the JSB Exact Series, chrony data at the muzzle and 25 yards at several different velocities from 500 to 1100 fps.... The pellets I used were the 13.4 gr. RS, 14.3 gr. Express, 15.9 gr. Exact, 18.1 gr. Heavy, and the 25.4 gr. King.... which all had remarkably similar drag profiles.... By using that, I have generated a new Custom Drag Profile in ChairGun specifically for that series of pellets.... What I did was keep adjusting the profile until I got the best fit for all my data, so that the BC is as close to a constant for each pellet over that velocity range.... Here is the new drag profile.... It is only valid from Mach 0.4-1.0, ignore the rest....

Using that drag profile, I redid the graph from yesterday for the JSB Exact Series..... Here is what I got....

You will notice that the drag increases faster over Mach 0.8 than using the model built into ChairGun Pro 4.... This shifts the curve to the left about 50 fps, putting the lowest wind drift at 850 fps, and the rapid rise starting at about 950 fps.... Assuming the considerable data I have accumulated is more accurate than the generic data in ChairGun Pro 4, then this suggests that for the least possible wind drift the best velocity range is between 800-900 fps, although I will continue to push towards 950 with my own guns just to get that little bit more FPE and flatter trajectory.... However, it does fit better with my experience than the previous graph which showed that velocities up to 1000 fps were not too bad for drift....

Other pellet shapes will produce different curves, but I think this is the best data I can provide at the present time for the JSB Exact Series....

Bob

Slavia · Moderator

Another justification for staying roughly in that .85 Mach region.