My high power shooting partner uses my handloads to good effect. Of course I tested his rifle to make sure they were in fact safe in his rifle (he has a Wylde Chamber, mine is a 5.56 chamber).
I wouldn't let a stranger do it, but it was another shooter who gave me 22 reloads of 80gr Amax's to shoot at 500 one time that convinced me to pick those bullets up for my 600 yard load.
I'm not sure about pistol reloads though, most pistol ammo is cheap enough that the risk/reward ratio for using someone else's reloads is not worth it.
Machine guns are awesome until you have to carry one.