At one time I would have said the 22 Remington Jet as I keep that cartridge and revolver on hand to defend against the illusive but deadly Chupacabra that has seen fit to migrate over here from the SouthWest.
However, these days the answer is far more complex. Everyone knows that government operatives working in the bowels of the Remington R&D department led the charge with the 22 Jet with Smith and Wesson back in the early 60's. But most people don't realize there was a second battle waged in the early 70's by this same group of intrepid bureaucrats. When the Remington group discovered that East German operatives had obtained a DNA sample from the Chupacabra, they knew there was more trouble brewing.
The East Germans apparently intended to breed a more powerful female Olympic athlete with this DNA ... and it's obvious that their efforts were somewhat successful. Clearly the Chupacabra is a hairless yet toothy foe that seeks to devour the hearts of young men. Does anyone think it's a coincidence that East German female Olympians were hairy, toothless, and sought to devour the hearts of young men? Yeah, sure ... just a coincidence.
Thankfully, the government insiders at Remington had the inside track in the early 70's, just as they had in the early 60's with the Jet. But this new beast was a relative of the Chupacabra ... and a European one at that. The old 22 Jet simply wasn't up to the task anymore. You know what I mean ... you've seen the pictures of those East German "women" back in the 70's.
The new solution required new thinking. And because of the European angle, it required ... the
metric system. That's when the 5mm Remington Rimfire Magnum was born. Obviously, I have a nice stockpile of 5mm ammo and an original Remington 592 with which to deal it out should any of these East German beasts find their way to Florida.
How do I know that these government agents existed and still fill this role today at Remington you ask? Two words ... "staggering mediocrity". Look at the Remington of today and tell me with a straight face that it isn't infiltrated with government bureaucrats determined to grind out overpriced, mediocre junk that's actually a regression from the stuff they made prior to WWII.
The only question is ... will they answer the call again some day when those crazy North Koreans try to interbreed a new weapon with the ancient DNA of that scourge ... the Chupacabra. I'm not saying I have inside information or anything, but I've been hoarding Jap Nambu cases for years ... just in case.

... I love poetry, long walks on the beach, and poking dead things with a stick.