[DesertRhino]

A screaming little rimfire that’s necked down for a 17 cal spritzer. Looks like a miniature 223. Common varmint round. Deadly at social distances, and very very nasty beyond.

[CopperTop]

Options are not limited to off the shelf offerings. Mebbe the guy's a reloading varminter and it could be a .17 Mach IV. A 20gr VMax at ~4000fps is a nasty l'il bugger at well more than "social distances". About the same for the .17 Remington. You can only stuff so much energy down that tiny hole at once, I reckon.
Owned both at one time or another. The Mach IV onna Sako Vixen and the 17 Rem out of a Thompson Pistol. Either was pure death on groundhogs. d;^)

Or mebbe it was a pellet rifle.