I’m mighty impressed with the RNS readers, as usual — Heartless Libertarian got it right off the bat, and without the benefit of my hints, which were posted after he guessed. Honorable mention to AughtSix, who gets bonus points for identifying the cartridge’s wildcat twin.
 
It is the Remington .17 Fireball, which is basically the wildcat .17 Mach IV in factory garb.
Remington has a history of “legitimizing” wildcats — the .22-250, for example –Â so this one was right up their alley.
It’s the cutest little cartridge I’ve ever seen.
It is a bit disconcerting to pick up the full-size cartridge box, though. It weighs about as much as the same box filled with empty .308 brass.
I now have a T/C Contender barrel in .17 Fireball, so I’ll be taking this baby to the range soon. In rifles, it’s ideal for the varmint fields, as there’s so little recoil you can spot your own shots. We’ll see if it works out that way in the pistol….
The Interweb is my friend. You said it was a factory round, so the green box ent me off to Remington’s website. Just rooted around until I found it.