Shouldn't the Barret .50 cal be illegal to use in this day and age since it would hurt to much?

There is no such rule in the Geneva convention banning certain caliber weapons as anti-personnel. Various military organizations do however have rules of engagement suggesting certain weapons should not be used against personnel but there's nothing illegal about using one. The rules are rather ridiculous. Possibly the worst round to get hit with is a .22. The link is rather graphic.