What is the smallest but fastest and most deadliest gun?

Answer 1

The 220 swift was the fastest commercial round until the 223 winchester super short magnum. It can push bullets to over 4600 fps. That makes it explosive on small game but it is still marginal for anything larger than coyotes. Bullets pushed that fast come apart and lack penetration for larger game where a heavier slower bullet that expands in a controlled manner would be more effective. Most deadly rifle round commercially available would probably be a large slow round like the 505 gibs or 577 nitro express.