For a light, fun, semi-auto, around $600, your 2 best options would seem to be the M14 or the WWII sidearm, the M1 Carbine. I have shot both and found neither to be what I, personally, would consider "accurate", but they're fun and will send lots of ammo down range.
When you throw the thought of hunting into the equation, everything changes. The M1 Carbine lacks the accuracy for varmints and the energy for hogs. The one M14 that I shot was perhaps capable of a better group than the 4-5" I got, at 100 yards, but that is nowhere near good enough for varmints. It might be suitable for hogs but I feel the .223 is best used by an experienced hunter/shooter, when it comes to pigs-gone-wild, because they just aren't as easy to kill as deer, requiring more precise shot placement.
So, what you are asking for is a tall order, indeed. You want something cheap, light, "fun", semi-auto and powerful enough to kill a hog, without too much recoil. I'm not sure such a weapon exists. The best I can suggest is an M1 just for fun-shooting and a more serious gun for actual hunting.