After making my air "rifle" and noticing how few seem to have made handguns, I put this together:



It's pretty much the same mechanism as my long gun; a ball-valve held back by the trigger and opened by the long spring.  Air chamber is the "grip".
Barrel is 1/2" aluminum tubing with a bore diameter of about 9mm.
I've been shooting 1/4" "split shot" fishing sinkers from Walmart patched with fleece material.   Works quite well!
Four strokes of the bike pump bring it up to 100 pounds, which seems adequate.
Need to make some sights!

No real way to measure velocity; I don't have a chrono... However I suspect it's substantial.  I just fired one of those projectiles at a weathered old 2 X 6 and the resulting dent was at least half the depth of the ball.
I could likely force this to 125 pounds without much difficulty, but the small air chamber means it gets hard to add air very quickly.

The Airgun Universe site has a conversion table for various sizes of buckshot...