Shotgunner, the cold, hard fact is, the best fit for what you want (semi, hi cap, cheap(er) ammo), is an AK. You should be able to get a Yugo for at or under $500. My goto rifle is a Yugo AK. Downside to them is they are heavy, since they are basically an RPK machinegun with a 16 inch barrel (meaning the front trunion is bulged, the barrel is heavier and the receiver is 1.6mm thick instead of 1mm thick). But, they do shoot good. I get 3-3.5 MOA with crappy Wolf, 2.5MOA with Winchester SP ammo.
If you want a little more PC, you can get an UNCONVERTED Saiga AK for about $350 or so.
If you add a bullet guide (easy to do, plenty of tutorials online, about $50, you can use regular AK mags instead of the Saiga specific ones. Conventional stock and non-AK front end, changes the profile a bit.
Good steel AK mags can still be had for $10.
Another vote for Gabe Suarez videos. The AK videos are great, and to be honest, the
tactics work for any rifle, not just AKs and are worth getting.
The
next best choice will be the new Mini-14s, but they MSRP for over $800, mags are expensive, and I don't think you'll find a new one in the price range you want. If you buy used, you risk getting one of the older ones with accuracy problems.
Another choice is an M1 Carbine, but good ones are over $700, ones for $500 can be had, but they are post-war knockoff models that may or may not be any good.
So, I'd seriously consider an AK, or go with a .30-30 levergun or milsurp bolt gun.