If you handload the 6mm has a definate advantage. To fully utilize it you'll need a 26 inch barrel. The 243 wont gain anything from a 24" to a 26" barrel but the 6mm does. neither of these are great for deer if you dont reload as factory loads are generally pretty weak(there may be be some premium loads around that are good though). Then again...a 22short is the perefect deer round if you have perfect shot placement every time.