Mr Blasty could be either Maggie Simpson's or Homer Simpson's gun, as Lisa is counting guns into a box named Maggie's guns, and realising one is missing, asks Homer for Mr. Blasty. Marge Simpson has also previously handed in a box saying Homer's guns.
Mr. Blasty gets melted up when Homer hands him in, and turned into playground equipment. Homer says "You'll be shooting angels now."
However, when Homer goes back in time to stop people handing in their guns, he succeeds, so Mr. Blasty would not have been handed in, so is still alive.