Guys,
Much of the confusion concerning BC, ballistic coefficient, is a function of two issues. One, BC is a variable that is dependant on bullet velocity. It changes dramatically in the transonic region, 1,350 fps down to about 1,100 fps. For HP bullets the faster they are shot the higher the BC when we are talking about the velocities those bullets are typically shot at. The second issue is that for 22LR bullets, typically launched between about 1,300 fps down to about 1,180 fps, the slower the MV the higher the average BC from muzzle to the ram line. That is why serious match ammo has a nominal MV of 1,080 fps so that it is not launched in the very turbulent transonic region.
The turbulence around the bullet in the transonic velocity region interacts with the wind to increase how much the bullet is moved in a given wind. Above about 1,500 fps bullet drag goes by the velocity to the power 1.5 and below about 1,100 fps the drag is about the same. In the transonic region the drag as a function of velocity goes by the 4th to 6th power, so the drag is very high therefore the BC plummets.
A very good way to think about the crosswind effect on 22LR bullets is to think about the vacuum-time-of-flight compared to the actual-time-of-flight. It has been shown that crosswind effect is a function of the difference between the VToF and the AToF, which is the time delta that the wind moves the bullet. VToF is just the time the bullet would take to get to the target if there was no drag, shot in a vacuum.
Here is a sample of the BC as a function of velocity for a 40-grain 22LR bullet.
Velocity BC
ft/sec [G1]
800 0.111
810 0.113
820 0.114
830 0.116
840 0.117
850 0.119
860 0.121
870 0.123
880 0.125
890 0.127
900 0.129
910 0.131
920 0.134
930 0.136
940 0.139
950 0.142
960 0.147
970 0.153
980 0.154
990 0.154
1000 0.154
1010 0.154
1020 0.153
1030 0.15
1040 0.147
1050 0.145
1060 0.143
1070 0.142
1080 0.14
1090 0.139
1100 0.138
1110 0.138
1120 0.094
1130 0.096
1140 0.097
1150 0.099
1160 0.1
1170 0.102
1180 0.103
1190 0.104
1200 0.106
1210 0.107
1220 0.109
1230 0.107
1240 0.108
1250 0.108
To continue here are actual testing results done a few years back with a chronograph to measure average BC over 100 yards.
First I acquired three different lots of 22 LR ammo. The first lot was Eley EPS, you know the new flat-nosed Eley match ammo that has a little dimple in the center of the flatnose. The second was Eley Match Extra with the standard match bullet. The final was CCI MiniMag. The reason for the two kinds Eley match ammo was to determine how the flat-nosed ammo compared to the standard round-nose match bullet that most 22LR match ammo has. I used the same chronograph for the near and far velocities. Thirty rounds were shot over the chronograph for each of the three types of ammo at the near, 1 yard from muzzle, and far, 100 yards, and the velocities were recorded. Summary statistics were calculated for the average velocity for each of the six 30-shot strings.
The testing showed some very interesting results. First the difference between a standard round-nosed style bullet and the flat nose at subsonic velocities. The average muzzle velocity for the Eley Match Extra, with a round nose, was 1058 fps. The average at 100 yards was 942 fps or about 116-fps difference. Remember the delta number when the high velocity ammo is discussed. When those velocity numbers, the near and far average velocity, are plugged into Robert McCoys exterior ballistics software we get an average BC over the course of fire, 100 yards, of about 0.181. Ive discussed 22LR BC with the chief ballistician at Federal. His company uses 0.141 as the standard. I suspect that is for their higher velocity ammo, which has a nominal MV of 1160 fps. The Eley EPS ammo had an average MV of 1069 fps and an average 100-yard velocity of 950 fps. The calculated BC is 0.184. That is not a significant difference between it and the round-nosed bullet unless one also includes the understanding that a bullet can be designed for a given rifling twist that will have a much higher BC because for the same bullet or maximum bullet length that will stabilize in a given twist, the flat nose will weigh more and therefore have a higher BC in the subsonic velocity range as almost all bullet drag in this range is base drag.
Now, onto the high velocity ammo testing. The CCI MiniMag had an average MV of 1274 fps and a far average velocity of 1009 fps or about a velocity loss of 265 fps, remember the number you were supposed to remember, 116 fps for the subsonic ammo delta? These numbers indicate an average BC over the course of fire of only 0.129, WOW!!! Faster is not necessarily better. I believe these tests translate to our sport very well. It is my belief that much confusion has slipped into match shooting that is based on high power exterior ballistics. Remember BC translates directly to wind deflection. The higher the BC the lower the wind deflection.
To summarize:
AMMO----------------MV--------100yd Vel------BC
Eley Match Extra---1058-------943-------------0.181
Eley EPS------------1069-------950-------------0.184
CCI------------------1274------1009------------0.129
I hear and I forget,
I see and I remember,
I do and I understand.
Heres to understanding!
