I do a lot of rough shooting and find that after a few hours the weight of my 12 bore over and under excessive. I have tried a lighter side by side but do not shoot so well with this format of barrels. A friend has recommended I try a 20 bore over and under as the weight of gun is similar to an English side by side. My questions are : What is the effective killing range of both bores?. What loss of shot spread can I exspect at ranges between 25 yards and 45yards?. I look forward to your reply.
It really all a question of shot load, not of gauge. Let’s say that you typically shoot 1-1/4 oz of #6 shot in your 12 gauge and 1 oz of #6 shot in the 20. If the vital area of a departing pheasant is 4″ x 4″ (just a guess), you have a 26.5″ killing circle with the 12 gauge and a 24″ killing circle with the 20. This is based on an optimum-sized pattern at the target (about 75% to 85% with #6s) and using the criteria of an 80% chance of a two pellet strike (same as 95% chance of one pellet or 50% chance of three) at the fringe of the pattern. In the real world, you might want to increase the fringe requirements, but for comparison it’s valid.
What it comes down to is that the twelve throws a bigger payload and this bigger payload covers a wider area. The diameter difference of 12 ga (1-1/4 oz) to 20 (1 oz) is 26.5″ compared to 24″, or on an area basis its 552 sq/in vs 453 sq/in. The 20 covers 18% less area than the 12 in this example. My goodness! That 18% is almost equal to the 20% difference in shot weight between the shells. Who would have thought it?
Note that I’m not doing this in terms of range, but in terms of pattern density at the same range. Each 10% change in pattern density is normally equivalent to about, sort of, one full degree of choke change and is good for an extra 5 yards of range. Each full degree of choke change gives the same pattern percentage (more or less, sort of) at each five yard increment. Example: IC is generally considered to be a 50% pattern, Modified generally a 60%, Full is over 70% (remember I said “sort of”- nothing to do with choke nomenclature is absolute). IC gives the same pattern at 25 yards that Modified does at 30 that Full does at 35.
Soooo- when you go from a 1-1/4 oz load down 20% to a 1 oz load, optimizing everything else, you will now be getting the same pattern density at 25 yards that you had gotten at 35 yards before. The reduction in shell size has cost you 10 yards in effective range. In exchange for an18% smaller pattern, you get a gun that is lighter to carry and faster to point. Whether the trade-off is worth it or not is strictly up to you.
You can always fudge the equation by using larger shot loads in the 20 or smaller ones in the 12. At a certain point pattern efficiency raises its head. A 20 will not pattern a 3″ 1-1/4 oz load as well as a 12 will pattern a 1-1/4 oz load, but the difference is less than the popular press would lead us to believe. I regularly get over 70% patterns from Remington Nitro Mag 3″ buffered 1-1/4 oz lead #4s out of my .015″ barrel of my 20.
Everything in hunting is a compromise. There’s no point in trying to hunt for long periods with a gun you can’t lift no matter how good that gun is ballistically. There’s also no point in hunting with a shot charge so small that you can’t humanely kill the prey. You want to find the happy medium. Being able to shoot accurately is VERY much more important than any discussion of payload and pattern. If you center the bird, it really doesn’t matter what kind of pattern you have. It’s only when you are operating on the fringe of the pattern that payload becomes important.
Generally, I think that the best advice I ever received was to hunt with the gun I shot best, not necessarily with the one that threw the biggest load of shot. The bird doesn’t care how hard you miss him.
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)