A Derringer is a velcro gun. (A lot like many .25 auto's)
Unless you have it pressed up against the intended soon to be departed you've found yourself opposite, you'll miss. I can't count the amount of time and ammunition I wasted trying to gain "competency" with a Remington O/U Derringer clone in .38 Special.
FWIW, it's the type of gun that will be fired a maximum of 3 times. Once, to see if it fires. The second time (God forbid) when you've found it necessary to screw it into some miscreants sternum or under his chin. And third, when the crime lab is trying to see if the ballistics match.
IMHO, practice pulling and cocking it in a timely fashion. Buy a 20 round box of the Magnums (because that's the smallest quantity you can buy them in) and call it good.
YMMV?