You can get upper assemblies for an AR series rifle thats built to fire a .50, but I dunno how accurate or expensive they are. And, watch gunshops for .50 rifles. They usually sell used for much cheaper than they were new, my local shop had one for $1,200 recently that would have sold new (scoped, bipod and stripped down for competition) for over $4,000.
Also, don't buy ammo at gun shows- you usually get what you pay for. Most of that cheap stuff is cheaper because of the extremely low quality of the components used, and/or because its bought in bulk from some third-world warehouse where its sat wet and unmaintained for decades. I've heard horror stories about that stuff, both the ammo and the guns. Buy from an established and licensed gunshop or direct from a manufacturer.
