Air Rifle Ballistics
There are a number of good air rifle ballistics programs available. In the past I have used the ballistics calculators of Hawke ChairGun, and American Air Guns. Nevertheless I always intended to check those against actual field measured ballistics and trajectory.
I ran a string line out to 70 metres and then marked the ten metre increments using my Leupold rangefinder which reads to 0.1 of a metre.
I positioned a table over the string line and then placed the rifle directly above the string line leading to the target. The muzzle was within an inch or two of correct position, but I would adjust that exactly as undertook the shooting of each group.
I also set up the tripod with the Chrony, making sure it could be adjusted over the range of the impacts without having to be re-positioned.
I measured the distance of the mid point of the chronograph from the target, to allow for the small impact of that.
Having set up my 97K on a solid sandbag rest I checked the distance from the muzzle to the target before taking each shot. The scope was an Aeon 6-24×50 SF on loan to me, for review, by Airgun Sports Australia. It is an ideal scope for the exercise, with sharp optics, fine cross hairs and an adjustable objective for parallax.
Having fired all my groups at the different ranges I measured the group centre points distance below the target centre. I looked up the H&N web site to get the ballistic coefficient for the FTT .22 pellet. That was listed as 0.019.
I then used the American Airguns calculator to generate the calculated trajectory. It was pretty close, but some adjustment was necessary to better match the measured field data to the calculated trajectory and velocities. I did that by adjusting the nominal zero point a little and then altering the BC until I got a good fit of real versus calculated data. That required me to move the BC from the nominal 0.019 up to 0.035.
With confidence in the fit of the calculator I then varied my zero distance to get a slightly longer range for hunting. The rifle had been shooting about 20mm (0.79 inch) high at 20 metres (21.9 yards). When I change back to me Hawke Airmax EV 3-9×50 AO scope I will adjust the sighting to replicate that trajectory with a zero range of 35 metres.
A Suitable Hunting Trajectory
When you consider the trajectory, velocity/energy drop off with distance, the decline in accuracy and the effects of wind drift then 40 metres is maximum hunting range as far as I am concerned. During the latter stages of my testing a slight breeze picked up. It was an eye-opener to see just how much the groups drfited off the vertical with that. I feel that the wind drift predicted by the calculator seems to be pretty much correct.
Actual Velocity Measured at each distance
Actual Projectile EnergyMeasured at each distance
Wind Drift at 40 metres for Varying Side Winds