Okay, the question then becomes, how do chain saws fail? They can stop running, in which case, no major problem. They can throw the chain, which can screw up bolt actions, and magazine access, but still relatiively easy to fix. Or they can snap the chain. If that happens, you have a strand of metal moving at a high rate of speed at and around the end of the barrel, damaging it and causing the weapon to not function and possibly blowing up in your hands the next time you fire it. Or it snaps around and chops into the breach, causing the weapon to not fire. Or it cuts into the magazine, damaging the rounds that are going into the breach, and possibly causing the gun to explode in your hands, or setting off the rounds in the magazine, causing the same thing. About the best thing you can hope for if it throws or snaps a chain is that it merely cuts off your lead hand. The weapon still functions and all you have to worry about is bleeding to death from the severed arteries in your hand. The chain saw is a cool idea in general, but in practice, I'd take a knife or bayonet on the end of my weapon over a chain saw any day of the week.
