Field 12 Gauge Lead
Field shotshells are used for hunting, while target shotshells are used to break clays.
Field loads are generally considered to cover shot sizes 6 all the way up to size T and include all of the buckshot sizes. Target shotshells generally start at shot size 7, and end at shot size 9. Of course, there are exceptions. Smaller shot sizes - such as size 10 - may be used to hunt certain game birds such as quail. It all depends on what you are hunting.
As there is much overlap in application of field shotshell/game combinations, when choosing your field load be sure to consider your game as well as the distance you are shooting from, and the payload of the round. The shot size of all products can be found in the description of each product along with the weight of shot loaded in the round.
The caliber of shotguns is measured in terms of gauge (U.S.) or bore (U.K.). The gauge number is determined by the weight, in fractions of a pound, of a solid sphere of lead with a diameter equal to the inside diameter of the barrel. So, a 10 gauge shotgun nominally should have an inside diameter equal to that of a sphere made from one-tenth of a pound of lead. By far the most common gauges are 12 (0.729 in, 18.5 mm diameter) and 20 (0.614 in, 15.6 mm), although .410 (= 67), 32, 28, 24, 16, and 10 (19.7 mm) gauge also exist.