Well, some folks laugh at me when I suggest the KelTec Sub 2000. I love mine, folds in half and slides right into my backpack and they accept the Glock 33 round mags. Mine is in 9mm, I can hit steel fairly consistently at 100 yds, and have done so at 200 but ya gotta aim at the moon to do it, lol..
They do have a threaded barrel for a can down the road. Oh, and their customer service is top shelf in my experience.
I've looked at a held a Sub 2000 a few years ago and it was interesting and has potential. The folding method negates the use of a mounted optic though, doesn't it? Not that it is a hard requirement, I'd prefer to have a red dot of some sort.