Originally Posted by EvilGenius
12ga is 12 ga, but there are 2 3/4", 3", 3.5" shells.
Hence 12 gauge Ain't 12 Gauge.
Depending on the load, and what you want to do with it, 12 gauges vary drastically.
12 gauge is about the most variable gauge, since the majority of international shotgun shooting competitions use it, but then again so do the majority of deer hunters in states that don't allow rifles, as do the majority of bird shooters, and the majority of people screwing around.
You can buy 12 gauge loads all the way from light target designed not to recoil much so that youth target shooters can shoot all day with out bruising or pain, all the way up to max DRAM magnum loads intended to drop geese at long range with heavy weight tungsten shot, all in 12 gauge.
Easiest way to think about it is backwards compatibility. Big heavy geese/waterfowl guns that have 3.5" chambers can handle most any loading wall the way down to light target loads (but the gun is big, and heavy, and swings slow, and if an autoloader may not cycle the light loads)
Light weight upland bird hunting guns from the 1950's will not shoot heavy modern loads, don't try it, not meant for it - much more fun to carry around bird hunting however.