I also been there thinkin of building a double on a .410 (for my doughter ofcourse) and as I have a 22 HP it would be obvious which caliber to use. But the "pencilround" 6x70r is so buitiful as a double round so I would probably go with that. And as its loaded for barrel inserts on combos and a lowpressure round(under 22 hornet) it woulnt be a problem. Also if the 22 HP comes hard to find for you it will be the same problem with the 6x70r.

The 22 HP does well on small game but as mention if you are a meathunter it can if you hit bone be a bit on the destroing end. But compared to the other 22 rounds suggested its much more usable on medium game if you intend to take care of it afterwords. As its not built as a crow blasting round.

Havent come across anyone using the 6x70r and dont know what bullets to use I cant ofcours recomend it but would use it myself.