An idea I had for a motorized Dalek gun using a motor similar to a steam engine piston.
The motor doesn't stop until it reaches the reset point with a leaf spring switch.
If you want a single shot than a full auto, add another leaf spring & toggle switch to select back to the full auto.
I made a quick drawing of the Resolution Dalek Gunstick from the Doctor Who episode "Resolution".
My best guess on what it looks like. The gun has only 4 ribs on the gun barrel, according to the screencaps. A finger guard that looks like a coat hanger hook, various beveled inner pipes, a blue indicator light & a 3 button like hand grip. Theirs a mushroom shape trigger button under the trigger guard... Ah.. Did I leave anything out?