I've had consistent poor results with manual wire strippers; if I use the nominal-sized hole several strands of wire always get broken or nicked, and if I use a larger hole I have to get a grip and pull far too hard. On short sections of wire I often find the strands having been pulled through the insulation, forming an irregular end.
The Sea-Dog Adjustable Wire Stripper & Cutter makes it easy to get perfect results every time. The way it works is that the jaws at the end grip the insulation staying on the wire, the sliding blades come together and start to make a cut into the insulation, and then the back pressure on the blades causes them to stop coming together and instead move towards the end of the wire while maintaining pressure; so it pulls the insulation apart rather than cutting it and produces a neat and square end.
I've used it on 12 to 22 gauge wire, without any adjustment in between, with equally perfect results.
There is a sliding stop for the wire end with a scale for setting the length of stripped end you want to make and there is a built-in wire cutter that works easily when the wire fits.