Hey Bruce,
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.
Al
Dear Al,
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).
Best regards,
Bruce Buck
Shotgun Report’s Technoid
(Often in error, never in doubt.)
Shotgun Report® 
If I understand the question it does not seem like you provided an answer. The question asked if 20 and 12 have the same size pattern, I believe that means diameter not pellet count or percentage based on choke. Perhaps the poster will clarify, if not I would like a response to that question?
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Dear Jim:
It’s always a question of definition of terms, isn’t it. If you
compare a 7/8 oz 20 gauge load of #8s with 359 pellets to a 12 gauge
1-1/8 oz load of #8s with 461 pellets, if both guns are choked
Modified and each throws a 60% pattern into a 30″ circle at 40 yards,
obviously the 12 gauge will have more pellets in the 30″ circle. Using
my faithful “Choke Chooser”, available from ShotgunReport.com, the 7/8
oz load will have an effective pattern on a standard trap view target
of 16″, while the 1-1/8 oz shell with more pellets will have an
effective pattern of 20″. The effective pattern is defined as the
circle in which the target will likely be hit by two pellets. So, the
pattern percentage size is the same (60%), but the effective pattern
is not (16″ to 20″) due to the difference in pellet count.
I hope that clears it up.
Bruce Buck
Shotgun Report’s Technoid
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