If Revolutionary War reenacting is in your future, then as others have said, you need to look for a used gun - probably a smoothbore. No Traditions, Lyman, CVA, T/C, or other rifle that you will find new for anywhere close to $600 will fit the time period. Do you know of any units near you? If so, find out what their members use, and if they are a reputable group, that will tell you what kind(s) of gun you need to look for. Many of these groups have people who have some items to help those just getting started "make muster," and every now and then, someone who is getting out of reenacting for whatever reason sells off his stuff; usually unit members hear about it first.
For all but the earliest days of the War, one of the variations of the 2nd Model Brown Bess would probably fit the bill. Used in good condition, they often run less than $600 for a Japanese or Pedersoli one, and while they will require a bit of tuning, they usually make fun guns to shoot, and they are passable for many units of either the Continental or British army. If there are no units that you plan to join right now, and you "need" a flintlock now with an eye to reenact later, that would be my call.