Good Shotgun Is A Realitive Term
I'm fairly new to shotgun sports, so this entire discussion is something I just went through in the past couple of years.
First law of shotgunning: The gun is the cheapest part of the sport. Don't obsess over the cost of the gun. Buy what you can afford and buy a gun that fits you correctly.
Second law of shotgunning: Buy a gun that fits its intended purpose. If you want a shotgun for a little hunting and home defense, economical shotguns work just fine. Look for sales at big box sporting goods stores. Here in California, Big 5 Sports often runs sales on entry level shotguns that often offer two barrels: 20 inch and 28 inch. Perfect for your home defense fantasies and hunting/sporting clay adventures. Mossberg, Remington 870 and any Browning will fit your needs and meet your budget.
Start with a pump. They tend to be the most gun for the money. Mossberg is a fine gun. I personally think you get more value with a Benelli, but that is my opinion. If you buy a used shotgun, ask about the warranty. Most reputable gun stores will offer a 90 day return policy or fix it for free. The semi automatic Remington 1100's are plentiful, easy to fix and parts are easy to obtain.
Third law of shotgunning: If the shotgun is knocking the snot out of you, you are holding it incorrectly or the gun doesn't fit you. Work on perfecting your form. Take some lessons. Figure out how to hold a shotgun and determine if you are right eye or left eye dominate. Go to a gun club or shooting facility that has a pattern board. Step off 20 or so paces and fire at the pattern board to see if what you aim at is what you hit.
Don't buy a shotgun that doesn't fit. Twisting the gun or excessive leaning to make you fit the gun will only lead to unhappiness. Here is a quick way to determine if a shotgun fits you: Assume your stance. Close your eyes. Bring the gun up to your shooting position. Open your eyes. If the barrel has two beads, you should see a clear figure 8. If it has only one bead, you should see the entire rib or the top of the barrel and the bead. If not, the gun really doesn't fit you. Put it down and move on to the next gun.
Fourth law of shotgunning: If you get into clay target sports, price of the shotgun does matter. Putting 5,000 or more rounds through your gun in a year takes it toll. Shotguns that cost less than $1,500 will start breaking down. Repair parts and their availability do matter when buying guns in this price range.
Fifth law of shotgunning: You haven't felt recoil until you start firing a Browning A5 in 12, 16 or 20. So if that shotgun is bucking or pounding you in the cheek, you are doing it wrong.
I shoot over 10,000 rounds a year using 12, 20, 16 gauge shotguns. I use factory loads and reload my own ammunition. I use steel and I use lead. I have hot loads and I have soft loads. And what have I learned? Gun fit is everything and the only thing. Experiment with slip on recoil pads. Not so much to absorb recoil, but to get the best LOP and fit you can.
I can shoot as well as anyone at my gun club with a 20 gauge pump Benelli as the guy with the $28,000 Perazzi. And sometimes I can catch the empty shell in the air before I take the second shot.
One last thing. No matter what you buy, get yourself a cleaning kit and a Bore Snake. Clean the gun once in a while and lub it as directed.