frogprince -

I think perhaps you're mixing two different things.

A 12 ga is sized so that twelve spherical lead balls the diameter of the bore of the barrel will weigh 1 pound. Another way of looking at it is that one lead ball the same diameter as the bore is 1/12 pound. A 10 ga is larger, so only 10 balls the diameter of a 10 ga are required to make a pound (with each ball being 1/10 pound).

There are relatively few 10 gauge guns around in my area, although they were much more popular some years ago, especially while black powder was still fairly common. They may still be used more elsewhere. 12 ga guns are by far the most common here, but my understanding is that smaller bores (larger numbered gauges) are more popular in places with less open hunting space.

The number of pellets (shot) loaded in a typical shell for a given bore size varies with the size of the pellets, but pellet size is to a different "system." Smaller numbers indicate larger pellets, but none of the common ones are larger than a "BB."

For "birdshot" - where the shot pellets are small enough to pour into the shell, if you subtract the "size number" from 17, the remainder is the diameter of a pellet in hundredths of an inch. For #6 shot, 17 - 6 = 11, so a pellet is 0.11" in diameter (APPROXIMATELY). For "buckshot" sizes that have to be "stacked" into the shell, a slightly different formula is used.

Commonly available shells also come in several lengths, with variations from about 2-5/8" up to 3" long (rarely to 3.5"), and of course the longer ones can hold a few more pellets of the same size along with more powder than the short ones. Increasing the total weight of the shot and the amount of powder to push it both lead to higher pressures, and for this reason some care in using only the appropriate shell lengths is required. You can't (safely) use a shell longer than the one a particular gun is intended for, although it is possible to use a shorter one of the proper gauge in many guns. (Some auto-feed guns are a bit finicky about the shell length, even if they might stand up to the pressures of "deviant" loads.)

It's also possible to leave out a few pellets in order to increase the amount of powder and theoretically at least to obtain higher velocity for the fewer pellets used. Within reason, this doesn't usually change the peak pressure by too much, but caution is advised if you're inclinded to "experiment." Some "different" loads are commercially available, or can be hand-loaded, but not too many variant loads are commonly used, so far as I've heard.

Especially since the advent of non-toxic (lead-free) shot, it's no longer possible to generalize absolutely that a shell of xx ga with #yy shot has exactly q pellets, although there's usually not a lot of variation. For a while after lead shot was outlawed for most hunting (in the US), much larger numbers of shot pellets for the lighter steel shot was common. The development of tungsten and tungsten/copper shot that approximates the density of lead has brought the "pellet counts" back down to close to levels that were common with lead, but there are still a few shell types that pack more or fewer pellets of a given size into the case. The label on the box the shells came in is the best way to resolve a debate about a particular shell, although you can also of course cut one open and count them.

John