I don't use spray paint myself. I use Duracoat with an el-cheapo air brush (paasche). Its more expensive than paint and requires tools like a compressor and the air brush. But it comes out mint and the finish is durable. The air brush applies a very thin and lite coat. Its a very smooth finish, and the thin coat generally doesn't impact tolerances.
I've done a shotgun for a friend, a shotgun for myself, and 2 ARs. Most of your time is spent on surface prep and degreasing.