I had to replace a broken belt on my old Cadillac a few years ago. Right when I cranked it up, my uncle says to me "did you find out why the belt broke?" To which I replied "huh?" Then he says, "well it broke for a reason..." SNAP!!! Broken belt. Again. Tensioner pulley was cracked and wobbling.
Are you sure everything else is in good working order? Unless you've put a lot of rounds down range, or have otherwise worn or damaged that spring, there's likely a reason it broke.