This may help a little - I wrote this in answer to a similar query:
Every once in awhile new shooters ask some basic shot-gunning questions trying to decide which shotgun they should buy. They are generally concerned with either home defense, or hunting, but often both.
Yes you can hunt small game and game birds with the big 10 gauge,but it is more recoil than most people want to deal with on a regularbasis. By and large the "big gun" is the 12 gauge. It has been a fight stopper from the old west to World War Two.
I have been asked many times if a 12 gauge is too much gun forhunting small game. There is a fear among some new shooters that the smaller animals will be too peppered with pellets to be edible or at best a gruesome mess. It is not true. Many, many, many rabbits and partridge and pheasants have gone down to the 12 gauge and been perfectly suitable for eating world wide over the last hundred years or so.
Sixteen gauge shotguns use a shell slightly smaller than the 12 gauge shell, but there is not a significant difference in the perceived recoil. In my opinion, the only thing gained by using a sixteen gauge is greater expense because the shells are used a lot less common and thus cost more than the twelve gauge or the 20 gauge.
That said, a 20 gauge is also perfectly suitable for all shotgun hunting and for self defense without the full recoil from the larger shells of 16, 12, or 10 gauge guns. I agree with the often repeated advice that new shot-gunners, most women, and younger teens who wish to hunt or shoot trap or skeet should begin with a 20 gauge. Many are built for smaller frame shooters (youth models) and they will probably be a better match for those folks.
The 28 and 410 gauge shells are much smaller than the 20 gauge and also more expensive. They can be used for hunting and defense, but it is like deer hunting with a 22 rimfire. These shells are really not best suited to the job. These smaller gauges are usefulf or teaching shotgun use, and for youths to hunt squirrels with, but in my personal opinion do not throw enough lead to reliably take birds on the wing. Others will disagree.
Once you decide on what shell to fire, the next question is what action choice to make for your shotgun. Single shots are simple to operate and inexpensive, but slower to reload and fire after the first shot than other types of actions. The venerable side by side shotguns aka "double barrels" are basically two single shot guns sharing a single stock. They are reliable in that you have two complete actions (triggers,hammers, chambers) so that if one breaks you still have the second, but also heavy. You are carrying two barrels as well. Pump guns are the next technological step. There are good ones and bad ones. They require two hands to operate and I have found them more prone to jam than any other action, but they did dominate the shotgun market for 50 years, so I really can't put them down too much. Any of these can and will work for you if you find one that you like and that feels natural for you to operate. I personally love the old outside hammer side by sides but they are not the optimum for efficiency. In my opinion that designation goes to the more modern semi automatic shotguns.
A semi auto will be more expensive than a pump, double, or singleshot but it will kick less because the springs soak up recoil. Follow up shotswill be very fast until you need to reload. The number of shells you can load at a time varies but is generally at least three so that you will get at least one more shot than the old doubleguns without having to pump the action or work a bolt.
If you have any concern with recoil get a semi auto. Regardless of what you buy, put a pad on the butt-stock to cushion your shoulder when you practice.
What does "gauge" mean anyway? As used here gauge means the number of round lead balls the same diameter as the inside of the barrel (aka the bore) that it would take to weigh one pound. It takes 12 lead balls the same diameter as a 12gauge barrel to weigh a pound. The smaller 20 gauge would require 20 balls of that barrel diameter to weigh one pound.
In general the number of pellets in a shotgun shell is greater for the bigger bore guns. A 12 gauge is a larger diameter shell than a 20 gauge. So when using the same sized shot, the 12 gauge will throw more pellets than the 20 gauge per shell.
Similar to the numbers used to designate gauge, shot size is also inverse to the number designation. The smaller the number - the larger the pellet. #8 shot is very small pellets suitable for bird hunting. #6 shot is a decent rabbit load. #4 and #5 is a larger pellet suitable for turkeys. #2 shot is suitable for goose hunting. #1 shot is big O, OO, and OOO are bigger pellets sometimes called buckshot. In general the larger the pellet, the more deadly it is when it hits large game. The trade off is that you fit fewer pellets in a shell.
A shotgun "slug" is one big pellet as big as the bore in your barrel (like a rifle bullet). For self defense any shogun shell will kill at close range, but #1, O, OO, and OOO are considered man stoppers.
You can fire shorter shells (like 2 3/4 inch length) in a shotgun that holds longer shells with no downside. But you can't fire longer shells in a shotgun designed for shorter shells. For example you can not use 3 inch shells in a shotgun with a 2 3/4 inch chamber. All things equal, get the longer chamber to increase your ammo options, but buy the shorter shells to reduce recoil. As a rule of thumb, the longer the shell the harder it will kick. So buy the 2 3/4 inch shells not the 3 1/2 inch shells. The short ones will do everything you want at under 50 yards.
This covers the very basics of shot-gunning terms and should give you enough information to start asking questions and narrowing down the choices when you decide which shotgun you want and what you want to feed it with.
I will add that my shotgun choice for home defense is the ultra reliable side by side stagecoach length gun with external hammers. Why? Because I can verify that it is loaded and cocked completely in the dark and I have had at least one failure from every other kind of shotgun over the years (semi, pump, and interior hammer break actions). If I am using my home defense gun I need it to be 100% reliable and the external hammer coach gun is the best option I know.
Good luck and be safe out there.
