Regarding the cartels getting those fifties... I seem to recall seeing a photo of some Fast & Furious guns, consisting of a crapload of AK-47s, several bolt action rifles, a freaking grenade launcher, and what appeared to be a .50 cal. So, odds are, at least some of those big guns are provided courtesy of the U.S. government. BTW, my understanding is that the Bush administration carried out a similar operation, so this problem has been there for a while.
Don't Border Patrol agents carry AR-15s while patrolling? If so, the proper response to a machine gun would be to run, then shoot the gunner from cover. Those big guns can't be easy to aim, and their ammo capacity is limited (250 rounds, I think), so it'll burn up pretty fast. An actual rifleman should be able to remove a single machine gunner without too much trouble.
Then again, it wouldn't shock me if our government in its wisdom decided not to provide agents with proper weapons. Quite frankly, it's insulting that the government treats its citizens and its own agents this way.
Fun fact: according to our superiors, we're supposed to just give a burglar/home invader whatever he wants, that way we don't get hurt. You heard that right: the proper responce to a violent break-in is to surrender. I, personally, am more inclined to give him a load of buckshot (or bullets from whatever gun is handy at the moment).