Take a look at the Rong-FU mill drill (http://www.use-enco.com/CGI/INSRHM) I got one several years ago from a machinery dealer. Someone had ordered it then backed out. I paid $1200.00 with tax. It has a 24 inch table and a travel of about 26". If you are only working on gun parts that should cover about 90%. I have machined a 24" black powder breach loading cannon and AR-15 0% forging. It takes Bridgeport collets and tool holders.I just installed a DRO which makes milling in the y axis a breeze.( the mill drills head goes up and down not the table) Got it on Amazon for 134.00 for three axis
The thing to remember is a used machine is just that, unless you can have it checked for wear and run out you might not be able to hold tolerances.
The other thing is a full size Bridge port weighs 3,000-4,000 lbs. and takes about up about a 5x5 ft. area.