I can make a lot of arguments against gun control, but since this thread is limited to the .50 (and I'm assuming the OP means .50 rifle and not handgun) I will keep my arguments to that.
If your government became a tyranny, and was going door to door taking people from their homes to be tortured to death in camps, which would you rather have to fight back with: a kitchen knife, dad's hunting rifle, or a big, nasty, loaded machine gun? Civilians DO have a need for a weapon as big as a .50. Not everyone needs to be armed with a machine gun in their garage, but people should have the means to stand up to their government if they need to. What if the police started to arrest people for unjust laws, or rounding "undesirables" up, could you stop them with a kitchen knife? Maybe I'm paranoid, but I think it's a bad idea to blindly and completely trust an armed group of people for your personal protection.
EDIT: In my hurry I forgot to add my main point :p What about defense of your country? Should that not fall on the civilian inhabitants as well at the established military? Yes, the military is better funded and equipped to deal with war, but there is a reason no one wants to mess with Switzerland.