20
by NuJudge
Things are a little more complicated regarding shot sizes. There are several different shot size systems. If you are not buying ammo for waterfowl, deer or home defense, the above comments regarding bird shot cover matters. Amongst the other shot size systems, which in some cases overlap, are waterfowl/swan shot, and buckshot.
Semiautomatic shotguns that are gas operated are sometimes not tolerant regarding shotshell length being shorter than the chamber. My brother has a 12 gauge 3 inch Remington 1100, and it will rarely function correctly with 2.75" ammo, and then only 2.75 ammo loaded to the highest pressures. With pumps, doubles and singles, cartridge length being shorter than the chamber is rarely a problem. With pumps, doubles and singles, a chamber being shorter than the cartridge case is a huge problem always.
CARTRIDGE LENGTH IS MEASURED AFTER FIRING UNDOES THE CRIMP. THE UNCRIMPED CASE IS USUALLY CONSIDERABLY LONGER THAN THE UNFIRED CARTRIDGE. A UNFIRED 3 INCH SHELL WILL FIT IN A 2-AND-THREE-QUARTERS CHAMBER AND FIRE, AND BLOW THE GUN UP. There are commercially available 12 gauge shotshells longer than 3 inch: I don't know if a unfired 3-and-a-half incher will fit in a 3 inch chamber, but LOOK at case lengths. Look also for 20 gauge shells getting mixed in with your 12s, as they can blow you up also.
Sometimes a shotgun will allow you to do lots of different things, each with different attachments or ammo. I have had several Mossberg 500 shotguns. Homely as a mud fence, but I had interchangeable-choke barrels for Skeet and Waterfowl, rifled barrels for ammo with slugs, I even have a muzzleloader barrel. With a 500, I get away with less forward allowance than any other shotgun I've used. Specialized ammo lets me also do things such as tighten or loosen shot patterns, as needed.
The mention of drams is a archaic weight measure for black powder, which for a long time was the only powder used. A particular number of drams of black powder, with a particular weight of shot, was generally associated with a particular muzzle velocity of the shot. Try to look at velocity, not Drams. Velocity of the shot, together with the size of the shot, is usually what determines what use the ammo is intended for. For close-in clay targets which you see in Skeet, velocities of 1100 feet per second are fine, and you want fine birdshot somewhere between #9 and #7-and-a-half, and 3/4 oz to a bit over an ounce will do fine. For most Trap games, the range is a little longer, so that 1100 fps may be a little low, you'll want more shot because your pattern is spreading over distance, and you'll want heavier shot to combat pellet energy loss with distance. For Pheasant, I need to force pellets through feathers, skin, and meat and break light bones, and for that I need much heavier shot (#6, or the more coarse #5), higher velocities yet (1350 fps is typically what I want). For a really tough bird like a Turkey I want a coarse birdshot like #2, but ranges are close so velocity is not that important. For ducks and geese, everything is different as the shot required is usually the less-dense steel shot, which does not penetrate and smash things as well, so extraordinary things are done to increase pellet energy, such as much larger pellets, and much higher velocities (I load mine to 1800 fps).
There is rarely a free lunch in anything. A light gun is a delight to carry all day, but if you shoot the kind of high volume of shells one does in Skeet/Trap/Clays, your shoulder will tell you things the next day. A heavy gun will be much more comfortable to shoot duck loads in, but if you take that gun grouse hunting all day, your arms will fall off at the end of the day. My experience has been that increasing velocity only increases recoil a little, but even small increases in shot weight dramatically increase felt recoil.
Some shotshells are made with cases that are not really intended for reuse. They are typically cheaper than those that are really intended for reuse. Those that you purchased can be reloaded, but typically are not. Those intended for reloading a lot of times typically cost something approaching twice as much.