A few years ago I made up a "Carry" shotgun. I bought a run of the mill budget singleshot shotgun, mine is a Stevens but others can be had made by H&R, NEF, Winchester and others, this type of gun can be picked up used for under 100 dollars, I paid 65 for mine. I then took it to a local gunsmith and had the barrel shortened to 18" the bead put back on, added a side ammo carrier and a sling. I would take pictures but as soon as my dad saw it a took it for his behind the door garden gun. The advantages are that it is short and very light, because of the weight I woud suggest a 20 gauge, 12 is brutal to shoot with heavy loads, the cost is very reasonable I think I have less than 150 in the whole project. If you are a little handy you can do the work yourself their are sites on line that will detail how. I know this is a single shot but with practice it can be loaded and fired fairly quickly, you can add rifle sites and improve the accuracy with slugs, I can keep them on a pie plate out to 30 yds with just the bead.
I have killed a lot of game with mine, it is dead simple and reliable, it has loads of knockdown with slugs, as someone else mentioned I prefer the Brenneke style slugs and have killed several feral hogs with them with good results. Switch to birdshot and it is an almost fail proof smallgame getter, and the best thing IMO is that it weighs almost nothing. Chris