When it comes to shotguns, I must plead ignorance. I'm specifically looking for a semi-auto shotgun for my wife and 14 y/o daughter. Readily available slug barrels and 18" home defense / riot barrels a must. Something simple to operate, and not all that complicated to strip and clean.
For the time being, I'm disqualifying pump actions, to reduce the prospect of short-stroking. Want a system they can become familiar with for bird hunting, but have alternates with the same manual of arms for home defense with slug barrels or shorter, unchoked barrels.
What would you recommend?
I'd recommend going back to the beginning and starting over.
It is interesting to note that you say you are not familiar with shotguns, but you have decided that your wife and daughter should use a shotgun for their defense and have decided what kind of shotgun they can learn to use (even though you don't know anything about using one).
You are condescending in assuming that your wife and daughter would not be able to handle a 12ga shotgun. But then again, if you keep telling the little women that they can't handle a real gun they will probably believe you and your idea will come true.
There are plenty of top quality reduced recoil 12ga buckshot and slug rounds available from most of the major manufacturers -- but your wife and daughter may surprise the hell out of you and be perfectly happy with full strength or even magnum loads. My daughter started shooting with an 1911 in 45ACP as a teenager, but her favorite gun to shoot is my Mossberg 590. She will happily run 8 full power rounds through it in under six seconds, then reload and do it again.
Unless your wife and daughter are physically handicapped, the possibility of short stroking is just a matter of training and practice.
If you really want an autoloader that was designed from the start to be meant for illiterate dummies, go with the Mossberg 930, which is similar to the 9400 that was originally designed for foreign military export sales (but the 930 is only available in 12ga).
If you have your heart set on 20ga, go with the Mossberg SA-20
http://www.mossberg.com/category/series/sa-20/
Any autoloader is more complex than any pump, but I'm sure if you show them how very slowly using small words they will eventually grasp the idea.