It's hard to beat a nail gun for perfect miters, especially if you're not skilled with a hammer. Trim nail guns allow you to hold the moldings in perfect alignment while you pin them in place. If you can afford only one trim gun, buy one that shoots thin 18-gauge nails up to 2 in. long. Fifteen- and 16-gauge nailers are good where more strength is needed, such as for nailing jambs, but the thicker brads make larger, more conspicuous holes and can crack thin moldings. Use shorter brads to nail the molding to the jamb, and long brads along the outside edges.