GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on November 21, 2017, 08:42:50 PM
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There have been many discussions about downrange ballistics, and since Helium has come on the scene, we now can play with velocities that were unheard of not that long ago.... I have done up some charts to compare what happens with a 100 gr. bullet with a BC of 0.200, compared to a 200 gr. bullet with a BC of 0.400.... The caliber is irrelevant, as long as it is constant, but because of the weights chosen, it might help if you think about a .308 cal powered by Helium for the higher velocities (or by air for the lower ones).... I used the G1 Ballistics Model, so think of these as flat-based Spitzers.... The BC is the important number, along with the muzzle velocity....
If you shoot a 100 gr. bullet at 1500 fps, that is 500 FPE, and if you double the bullet weight, about the best you might expect is about 1100 fps, which is 537 FPE.... Conversely, if you tune the gun to shoot a 200 gr. at 500 fps, you might hit around 700 fps with a 100 gr.... That is why the charts for the light bullet use a range of MVs from 700-1500 fps.... while the charts for the heavy bullet show a range of 500-1100 fps.... I am simply trying to get a valid comparison between a light bullet driven fast and a heavy bullet driven at just slightly higher FPE, just as you might achieve in reality.... Anyways, here are the charts for what is happening at 200 yards downrange in a 10 mph crosswind....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Downrange%20100%20VE_zps8fijkr1i.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Downrange%20100%20VE_zps8fijkr1i.jpg.html)
Note that even starting at 1500 fps, the bullet is subsonic at 200 yards (it has lost 450 fps).... and has lost just over half it's energy.... However, if you start it at only 1000 fps, it has only lost 150 fps, and retains over 70% of it's energy....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Downrange%20100%20DD_zpsfwuslewo.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Downrange%20100%20DD_zpsfwuslewo.jpg.html)
In the second chart, the bullet drops about twice as far starting at 1000 fps than it does at 1500 fps (80" instead of 40").... but it only has about 10" of drift instead of 15"....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Downrange%20200%20VE_zpsuxtw0ur1.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Downrange%20200%20VE_zpsuxtw0ur1.jpg.html)
By comparison, the heavy bullet (with it's superior BC) retains 79% of its energy starting at 1100 fps, and 86% starting at 900.... It has only lost about 120 fps when starting at 1100 fps, and only about 80 fps if launched at 900....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/Downrange%20200%20DD_zpsygh1apwe.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/Downrange%20200%20DD_zpsygh1apwe.jpg.html)
In the last chart, you will note that for a given muzzle velocity, the heavier bullet, with its superior BC, has a flatter trajectory compared to the lighter bullet in the second chart.... In fact, if you compare the heavy bullet at 950 fps to the light one at 1000 fps, they have the same drop.... and the heavy bullet has 5" of drift, compared to 10" for the light one....
My personal preference is to use whatever bullet your gun will shoot in the 900-1000 fps range.... I know that some prefer a lighter bullet moving faster, and that's fine too.... All I am trying to convey here is what happens if you take that into the Supersonic range, where the drag dominates bullet performance.... Yes, you can get a flatter trajectory, as after all, velocity is the dominant factor in determining that.... much moreso than the BC.... There are compromises to be made, however.... and hopefully the above will give you something to ponder to determine what those are....
Bob