Sighting an Air Rifle Scope – Theory
Firstly, if you are shooting a spring powered air rifle you must have a scope that is specifically designed for such use. Spring powered air rifles have a to and fro recoil that you do not get on conventional firearms. It is the forward jolt, as opposed to the actual recoil, that will shake your reticule loose.
For this reason scopes that may have functioned flawlessly on heavy kicking big calibres will quickly fail when attached to a big spring powered air rifle.
My Weihrauch HW 97K is less than eight months old and I am just about to sight in my third scope on this rifle. The previous two scopes, which the manufacturers stated were designed for use on air rifles, both had reticule failure.
The first scope was a Tasco 10×40 which had seen some service on another big springer air rifle, following an initial 4 or 5 years use on a 22-250. The reticule may have already been loose from that previous air rifle, because I had problems with this scope on the Weihrauch pretty much straight up.
After some research I bought a new Bushnell Trophy 4-12×40 AO for the Weihrauch and everything looked pretty good for a few months. Gone were the sudden changes in point of impact and group sizes improved significantly. The honeymoon lasted a few months and it was soon apparent that the Bushnell had dropped its reticule as well.
Further research pointed me at the Leapers scope range which are said to have a good trouble-free reputation on big springer air rifles. I had considered the Leapers earlier but had opted for the Bushnell because it had the same duplex reticule as my hunting rifle scopes and it was supposedly able to handle big springer recoil. It was mostly the mil-dot reticule on the Leapers that pushed me to the Bushnell.
Anyway, I will soon be the proud owner of a Leapers 3-9×40 and, if it functions as well and reliably as I am encouraged to believe it is capable of then I will learn to love the mil-dot. Another feature I liked about this model Leapers was that it has its parallax set at 35 yards (=32metres) which is a typical mid range for an air rifle and, as on hunting rifles, can then be ignored in shooting, if you are not a competition target shooter.
I am looking forward to resuming my testing of different pellets in the Weihrauch. I had given up on that when it was obvious there a scope problem. From what I have seen myself, and read elsewhere, the Weihrauch HW 97K is capable of some very fine accuracy, if you feed it the pellets it prefers.
Again, my initial experience mirrored that of other Weihrauch shooters; my rifle seemed to have a definite preference for the H&N Field Trophy Target 14.7 grain (0.95 gram) pellets in .22” calibre.
While I was waiting for the Leapers scope to arrive in the mail, I decided to investigate the theoretical ballistics of the rifle and pellet combination. For that I sort the Hawke Chairgun Pro calculation which is specific for air rifles. I had previously chronographed the H&N FTT pellets out of my rifle and found that they deliver an average 660 fps (200 m/s) velocity. The Hawke Chairgun Pro program is quite useful for exploring air rifle ballistics, but seems to have disappeared off the internet. Luckily, I had a copy on my spare computer and was able to resurrect the application.
I decided to work in with the features of the scope by zeroing the sighting at the parallax distance of 32 metres (35 yards). Note that the other zero point is at 8.1 metres (8.9 yards). That provides a trajectory with apogee at 22mm (0.88 inch) above the line of sight at 20 metres (21.9 yards). The trajectory curve for that is shown below.
So, the key ranges for testing will be at 10, 20, 32 and 40 metres. The trajectory curve really confirms what common sense would tell you, that is, the effective hunting range is out to 40 metres, with 10 to 32 metres being the prime distance for hunting. Further confirmation of that is in the pellet energy table, as follows, which shows how the 14.2 ft-lbs energy at muzzle velocity drops below 11 ft-lbs at about the 40 metre mark.
On a calm day, with the rifle and scope shooting really well, you might push out towards 50 metres, but that is pushing it and the pellet drop rapidly becomes pronounced.