I understand your pain and discomfort. I have been doing a lot of research and experimentation trying to find something comfortable.
The suggestion on switching to "reduced recoil" shells is a very good suggestion. There is a whole lot less kick, but from my tests they seem to be just as lethal/effective.
The other issue is, does the gun fit you? Is the stock too long? Too short? Is the angle/drop at comb wrong? This is easy to check. Shoulder the gun in your shooting position, and observe how much of the recoil pad is making contact with your body. The less the pad contacts, the more energy is concentrated into a tiny area (painful!). If the stock is too long, you are put in an awkward position, and this also results in undue pain.
The remedy for this is another stock. If the original stock is too long, there are countless options in shorter length stocks. I'm tall so most stocks are too short for me and the angle is wrong. I have switched to pistol gripped buttstocks and they work well. They have a slightly greater angle so the recoil pad is lower. Another advantage is the pistol grip transfers some of the felt recoil to my hand from my shoulder. This helps tremendously.
I have also increased length by using the slip-on recoil pads. The slip-on pads are soft so they contour to my shoulder and the whole pad makes contact. I recently tried one of the SVL (Sims Vibration Laboratory) LimbSaver recoil pads. Wow what a difference! I put it on a little NEF youth 12gauge ( this has got to be the lightest 12 gauge in existence). With the LimbSaver pad I fired full power 2 3/4" buchshot loads with no discomfort at all. None. I usually have a bruised/marked-up shoulder from any 12gauge I shoot, but not this time. I fired some reduced recoil slugs too. I'll try full powered slugs next time.
While we're talking shoulders, I'll mention shooting position. When you are shooting, swing your strongside elbow out. Don't have it low or tucked into your ribs. There is something about swinging the elbow out that puts the muscles in a better position to take the recoil. If there are any shooting instructors reading this, they'll back me up on that.
Back to your original question, there is absolutely nothing wrong with a 20gauge for defense. The Winchester Defenders are excellent guns. I don't have any experience with the 20gauge Defender, but I am building my own from a 20gauge Ranger 120 (youth model). The 20gauge Defender is only 4 oz. lighter than the 12gauge, so weight is negligible. If you shed 4 oz. in a handgun, that is a major weight reduction, but the Winchester Defenders are 6lbs 8oz for the 12 and 6lbs 4oz for the 20. You need to change weight by the pound in a shotgun to feel any difference.
One of the good reasons to get the Winchester 20gauge over any other brand is that it accepts all the aftermarket accessories and stocks that the 12gauge does. That is not the case with Mossberg or Remington. That is important because there are little to no accessories made for anything other than 12gauge guns.
I don't know if you would have to use 3" shells in the gun. I have had good success with 2 3/4" 20gauge ammo. Ammo is something you need to consider. There is a huge and wide selection and variety of 12gauge ammo. If you close your eyes and imagine something insane, chances are somebody makes it in 12gauge. But the selection in 20gauge defense loads is somewhat limited. Do some ammo shopping before you finalize your decision.
I touched on a myriad of items, but I hope it helps.
2guntom
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