I've never bothered to chronograph them, but these .25 ACP loads with the single ball probably do about 500 or 600 feet per second. A slingshot can't match that. I chronographed a Wrist Rocket with ball bearings some years back, owned by a friend. He got 300 fps at most, and usually downaround 250 fps.
I use the .310 Hornady ball in .32 Long Colt loads for my Marlin Model 1892 lever-action rifle in that caliber. The ball is too large for the case's .294 inch interior diamter, so I seat it firmly on the case with the blunt face of the expanding die. Looks terrible, but the case mouth pushed into the soft lead holds it in place. A dip of the seated bullet in Lee Liquid Alox provides lubrication.
This .310 ball load is accurate to about 25 yards, then the groups widen. Beyond 25 yards, it has the trajectory of a thrown grapefruit.
But the Hornady .310 ball is readily available and very useful in .30-caliber rifles and .32-caliber handguns. In rifles I'd suggest a light charge of Unique, or perhaps Trail Boss. In handguns Bullseye, W231 or HP-38 would be good. I haven't tried Trail Boss but the above poster says it works fine.
In my Marlin .45-70, I've loaded up to three .457 balls, separated by a thin card wad, in one case: one shot, three holes on target! This is adequately accurate for home defense out to 50 yards.
Every once in a while a factory produces .38 or .357 ammo wih multiple projectiles. This can be duplicated with a .375 inch ball sized down to .358 inch. It's usually advertised as a law enforcement or home defense round. In the early 1970s, I recall hearing that the Sky Marshalls carried Charter Arms .44 Specials with multi-ball loads. Two .44-caliber balls in your torso would ruin your day!
The humble lead ball is a tinkerer's delight if you reload. It's versatile, inexpensive and often readily available.
"And lo, did I see an ugly cat. Smoke. Brimstone. Holes in parchment. And this ugly cat was much amused." --- The Prophesies of Gatodamus (1503 - 1566)