It really depends on what you want to do. For barrel work, there are two approaches - a spindle bore larger than the biggest barrel you would pass through it, or a c to c length greater than the longest barrel you would work on, plus reamer, etc. A spindle bore of 1 1/2", with a c to c of 36" are dimensions that would work well. You can get away with shorter if the spindle bore is large enough, or with a smaller spindle bore if the lathe is long enough. Generally speaking, bigger is better. Most gunsmith milling can be handled with a mill-drill, or even a milling attachment in a lathe. As far as make goes, that question will result in a lot of discussion. I have done most of my machine work with South Bend lathes: 16" w/ 57" centres, a heavy 10, w/36" and a 10K (light 10) w/36". The 10K had a small spindle bore, so the length was important.