I believe that each caliber should have at least 2 types of birdshots, and this would affect the number of pellets fired and/or the speed of the shot. If it is a small number of pellets and low weight, it will be faster than if you shoot with a larger pellet with a high weight.
It should be noted that if for a caliber there are two shot sizes but with the same weight, the larger size will be slower than the smaller shot.
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Cal. 10:
- nº3 - 42gr
- nº4 - 36gr
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Cal. 12:
- nº5 - 34gr
- nº6 - 34gr
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Cal. 16:
- nº6 - 30gr
- nº7 - 28gr
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Cal. 20:
- nº 7 - 28gr
- nº 8 - 26gr
The same should be done with bullets, adding different grains, for example 2 different grains to each type of bullet of the one caliber.