I know some one who bought one of those H&R single shot break action shotguns. With this idea in mind to have , a low dollar solution to being able to use all types of shot gun ammo. Aside from 10ga. His thing is "shot guns will be the most useful and important post SHTF gun". So he got an H&R single barrel break action shot gun and barrels to fit his H&R in all the "uninteresting gauges". Since he has several 12 gauges there was no point in getting the 12ga barrel so he got the 3 inch chamber 20ga H&R shot gun and sent it off to get fitted for 16ga, 28ga and 410 barrels. That way if he comes across any non 12ga ammo he can use it as is. I think he said the 20ga was $75 in 1998, then each barrel was $40 plus one time shipping to and from the H&R factory. I think the prices have gone up marginally since then. I still see those H&R shot guns and I think they are between $100 and $120 now.
I see his thinking on this, but personally I think a person would be better served sticking to one gauge and stocking up ammo. If you were dead set on a single shot, why not get it in 12 gauge and then buy the sub calibre devices to allow one to use the smaller gauge shells?
I think he likes repeater 12 gauge shot guns, as far as I know he doesn't have any type of break action 12.
Sorry, but if I'm down to scratchin' for .410 or 20ga shells, I'm using my bow. If I have to carve a bow and arrows, that's doable too! Now, I use 12 ga, and I will probably die of old age before I run out!
The Handi Rifles and Shotguns are actually quite nice. Remington took them over a while back and closed them down. I have several of them and a .410 and 20 gauge have their uses and each have some brutal loads available now days. I pretty much only squirrel and rabbit hunt with the .20 gauge a lot less pellets to pluck out. Takes about a minute to change over to the rifled slug barrel. Picked up 5 handi rifles in one swap .308 30-06 .300WM .22M and a .243 all very nice and very accurate and very basic. And they were very low cost.