Watch out for Uruguay--drive times can be one to two hours each way (Trek). Also watch out for the places that give you a bunch of "free" shells or are too cheap to be true; there may be a reason. Also, there are a bunch of must brings: gloves, recoil pads, rain gear etc. Either Uruguay or Argentina is easy to bring guns (and you should in order to have the best time, even if you bring your own autos). I would also watch out for places that make you pay in cash (as opposed to credit cards) for shells and incidentals.
