A double barrel shotgun, a gatling gun.
There are .22lr Gatling guns, and not the stupid thing made out of two 10/22's but a real rotary Gatling action. They're not cheap. I really want one.
And a double-barrel shotgun could use chamber inserts, couldn't it?
===
I'd also love a .22lr pump or lever action that loaded from the breech like their center fire counter parts!
Heeled bullets loosely seated in rimfire cases are not a good idea for loading through a spring loaded and angled gate into a spring loaded tube.
Maybe, with a gate that flipped wide open rather than being spring-loaded,
maybe you could get it to work safely in .22wmr or one of the .17 calibers.
A pre-loaded speedloader makes shooting the Henry more fun, but the fun factor outweighs the tedium of loading from the front for me, esp. with CCI CB shorts.
===
I would buy a pocket pistol that came with a ,22lr conversion - the conversion for the KT pf9 is pretty slick and I'd REALLY like one for my KT p32 and the Mrs's SIG p238. For that matter, it would be nice to shoot .22lr in a colt 1903 pattern gun, just for ammo cost and having modern production of that superb design.
I'd like an XD conversion, for that matter.
I have a purpose-built 1911 conversion from Armscor.
I have a fake M1 Carbine built form a 10/22 (the Chiappa version is still in teething stage, I'm hopeful for a winner in a few years)
And speaking of Henry, I'd like a REAL Henry pattern rifle in .22lr, and/or a Volcanic replica (they'd have to add an entire extraction/ejection system concealed somewhere for that)