I finally had a chance to get out and measure the Ballistics Coefficient of the .250 cal 51 gr. BBT from the mold that Veral at LBT made.... I used my Disco Double, which has a 14" twist TJ's liner with a .250 groove and .243 bore.... The muzzle velocity was 873 fps and the velocity at 50 yards was 824 fps, so the BC works out to 0.125.... This is 30% better than the .25 ACP 53 gr. Round Nose made from the Lyman # 57902 mold, which started out at 868 fps and was going 804 fps at 50 yards, giving a BC of 0.096.... The improvement in BC will reduce the wind drift at 100 yards by over an inch.... and compared to a JSB King the drift is reduced by 70% from nearly 12" to well under 4" in a 10 mph crosswind.... In addition, the BBT retains 80% of it's energy at 100 yards compared to just 50% for the Kings.... The rebated boattail design is doing just what it was supposed to....
I only have one data point.... Here is the quote from that post....QuoteI finally had a chance to get out and measure the Ballistics Coefficient of the .250 cal 51 gr. BBT from the mold that Veral at LBT made.... I used my Disco Double, which has a 14" twist TJ's liner with a .250 groove and .243 bore.... The muzzle velocity was 873 fps and the velocity at 50 yards was 824 fps, so the BC works out to 0.125.... This is 30% better than the .25 ACP 53 gr. Round Nose made from the Lyman # 57902 mold, which started out at 868 fps and was going 804 fps at 50 yards, giving a BC of 0.096.... The improvement in BC will reduce the wind drift at 100 yards by over an inch.... and compared to a JSB King the drift is reduced by 70% from nearly 12" to well under 4" in a 10 mph crosswind.... In addition, the BBT retains 80% of it's energy at 100 yards compared to just 50% for the Kings.... The rebated boattail design is doing just what it was supposed to.... Here is the drawing for that BBT....I ran it through the Kolbe Drag Calculator, and here is the results.... The discontinuities are a result of limitations of the program, but are just above Mach 1 anywayThis is the type of drag model I wish to develop, but from measured data, not derived from calculations.... The only way to do that is to shoot it at various muzzle velocities and measure the downrange velocities....Bob