Your price point buys you a top of the line Remington/Mossberg pump 12 gauge or a low er end semi-auto. Everyone has an opinion on this discussion so really you just need to see the options first hand. It's hard to go wrong either way.
I started with a Remington 870 Express Talo and it was awesome. Never an issue shooting all flavors of 12 gauge ammo. I inherited a Mossberg 500 Mariner and thought I didn't need the 870 anymore - selling it was a mistake - even though I got a great price as it was during the gun frenzy of the Obama election. I started having issues with the 500, the pump tends to jam on the extract process. I will need a gun smith to investigate but I have lost confidence in it as a HD tool.
I recently picked up the Mossberg 590a1 you see below (walnut stock is beautiful) and it is awesome. You should be able to find one for ~ $600 brand new. The one I bought came with a bayonet - for zombies of course.
