I have a very basic question in regard to shotgun pattern in terms of 12 ga. vs 20 ga.(or any other ga. for that matter).
It is my understanding that the pattern size of a 20 ga. at 30 yards would be the same size as a 12 ga. pattern at 30 yards and that the shot velocity would be the same. The difference being the the 12 ga. pattern would have more shot inside the pattern than the 20 ga. pattern, just how much more shot I’m not certain.
Of course the above “assumption” is based on the fact that both guns have the same choke, the shells have the same size shot, the shell load is the same in comparable terms, and the barrel lengths(which is probably negligible) of both guns is the same.
Your question is a common one and it never hurts to cover it every now and then. Here’s the deal on comparing patterns in gauges:
If we ignore the .410 bore (which uses a closer distance and smaller circle), all other gauge patterns are measured as percent of the total shot load within a 30″ circle at 40 yards. The size of the shotload doesn’t matter, just the percentage of the total.
For example, Modified choke designation indicates that it achieves a 60% pattern in the 30″ circle at 40 yards. 60% of 3/4 oz of #9s from a 28 gauge is a Modified pattern just as 60% of 1-1/2 oz of #4s from a 12 gauge magnum. 60% is a Modified pattern of 60%. The actual pellet count is NOT a factor. Just the percentage of the total.
Now obviously, if you are using the same size of shot but with different weights of payload, the densities of that 60% within the 30″ circle will change. A 60% modified pattern of #8s from a 3/4 oz 28 gauge is clearly not going to have as many pellets as a 60% modified pattern from a 1-1/8 oz 12 gauge shell. There aren’t as many pellets to start with. 60% of a small number is always smaller than 60% of a larger number. It doesn’t always depend on gauge either. A 60% pattern of #8s from a 1-1/8 oz 20 gauge magnum will have a higher pellet count than a 60% pattern of #8s from a one oz 12. The math is even easy enough for me to understand!
One other question that you seem to ask: gauge has nothing to do with pellet velocity. Larger gauges give you more options as to what velocity you can use because they have extra space to play with, but the gauge itself doesn’t dictate any particular velocity. I load all four gauges to about 1200 fps for all my shooting except steel (which I don’ t reload).
Shotgun Report’s Technoid
(Often in error, never in doubt.)