Clay Target Q&A
With Russell Mark, Olympic gold and silver medallist
Questions: russell@corporateshootingstars.com.au
Q I need to settle an argument I have with a longtime shooting colleague. He shoots a 28" barrel over-and-under shotgun and I shoot a 30" barrel. We both want our firearms to shoot higher at their point of impact, as we intend on shooting some Trap targets. He believes that because I have a longer-barrel gun, I will need to raise the comb of my stock more to have it shoot as high as his. Is this correct?
Bill Anderson, Qld
A Mathematically, Bill, believe it or not, your friend is correct - ever so slightly I must add. I will try to explain this to you in the form of a simple equation based on some fairly basic assumptions.
First of all, you need to measure the distance from the end of your barrel (the shotgun’s front-sight) to your eye (the shotgun’s rear-sight). Let’s keep the measurements in metric and assume for you, Bill, that the length between the front and rear sights is 100cm, which is fairly normal.
Let’s assume your friend has an identical shotgun, apart from the 5cm shorter barrel; therefore, his length is 95cm. We assume also that you are using the same ammunition, that your eye placement is above the stock, that there is no effect of gravity on the shot, and so on.
We next need to know how far out from the end of your barrel you are hitting the Trap targets. Most Trap targets are shot at around 30m and to keep the measurements consistent, this equates to 3000cm.
We now can calculate a simple sighting plane factor by dividing the target-breaking distance by the length of the sighting plane. Bill, your factor becomes 30 (3000 divided by 100) and your friend’s becomes 31.6 (3000 divided by 95). From here, all we need to know is how far you intend to lift the comb of the stock to make the shotgun shoot higher. Let’s assume it’s 3mm (0.3cm). To find the point of impact at 30m, we simply multiply the desired new comb height addition by your sighting plane factor. For you, Bill, it is 30x0.3, which equates to 9 and your friend’s will be 31.6x0.3, which equals 9.48.
In layman’s terms, this means at 30m, the longer barrel will shoot 9cm higher by adding 3mm to the stock’s comb height and the shorter 28" barrel will hit 9.48cm higher at the same distance. Bill, to have the guns shoot identically, you will need to raise your comb by 3.16mm, as opposed to your friend’s 3mm. If you can pick the difference in comb heights, then you are a better man than I am!
This formula holds true and is a useful tool if you know exactly where you want your gun to shoot at a nominated distance. A good rule of thumb that this formula is well used in conjunction with is that for every 1mm you change your stock, you will alter your pattern by just less than 4 per cent. Less than 0.5cm difference in your point of impact at 30m with a shotgun is not even a debatable point in my opinion, Bill, but in answer to your question, you owe your friend a drink when you are done shooting next time.