Airgun Guru: what happens to a pellet in flight?
PUBLISHED: 10:17 10 February 2017 | UPDATED: 10:17 10 February 2017
A reader is noticing that the pellet strike is changing depending on the distance, but isn’t sure what is happening
Q: I am using a Gamo Phox .22. I find at short-range - 10 metres - the pellet strike is centre to aiming point. At 25 metres, I notice that the strike is higher, at around 30mm to POI (Point of Impact). I would expect the pellet to drop over distance, as it did when I was using 7.62 centrefire, up to and over 1000 yards. I’m using Accupel Domed, and Crossman Ultra, having discovered this rifle does not like the Panthers that were excellent in my Crossman Phantom. Can you explain?
The Airgun Guru says:
It’s true that pellets drop with distance, and that’s why, when zeroing, we raise the angle of the barrel to counteract the effect of gravity, but that only works out to a particular distance. From the muzzle to the apogee (the point at which the pellet is at its highest above the sight line), the pellet rises, and past the apogee, the pellet falls.
In your case, the pellet leaves the muzzle and rises toward the sight line until it crosses it at 10 metres (your near zero), but it then continues to rise relative to the sight line until it runs out of steam. Gravity is able to stop it rising further, which seems to be in the region of 25 metres (the apogee).
Beyond the apogee, the pellet will start to fall relative to the sight line, until it crosses the sight line a second time (your far zero), and then it will continue to fall the longer it’s in flight.
The same principles apply to the 7.62mm rifle, but because the projectile is travelling many times the velocity of a .22 pellet, and because it maintains velocity better in flight, it reaches the target much more quickly. The drop is a function of the time the projectile is in flight (flight time in seconds squared, multiplied by 192 gives the drop in inches), so it drops much less than any airgun over a given distance. So much so that you appear to have been unaware that it rises through the sight line and continues to rise to the apogee, but were only aware of increasing drop at much longer range.
I wholeheartedly recommend you download a copy of Hawke Chairgun Pro. This excellent free application allows you to enter data for your rifle, scope, pellet and zero range, and gives both a graphical illustration of the trajectory and a table showing where the pellet is relative to the sight line at 10-yard intervals. This will allow you to find a zero that limits the height of the pellet above the sight line at the apogee.
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