Lets look at some charachteristics of 50 cal. rifles, shall we?
1) They weigh upwards of 20 lbs.....not exactly transportable without a vehicle.
2) The ammo cost is over $1/round......why would a terrorist want to spend that kind of money on ammo?
3) A 50 cal rifle runs anywhere from $4000-$12,000.
4) A terrorist can obtain an AK-47 for less than $100 outside the U.S and buy thousands of rounds of ammo for what it would cost for 200 rounds of 50 ammo. Why would a terrorist bother with a 50? Common sense says they won't! This is a classic campaign of propoganda based on ignorance that the sheep will take as truth, hook, line, and sinker.
BTW, your typical enthusiast is a white male, middle aged, with an income over $75,000. Does this sound like a terrorist to you?
Seems to me all of the above charachteristics would be very unattractive to a terrorist.