A .22 will do the job.

In Verona, Kentucky, a 92 year old WWII vet was awakened by someone kicking in the door to his basement. He'd previously been the victim of burglary and theft from his farm. He got up, grabbed his .22 "squirrel gun", and sat down in a chair to await who might come up the basement stairs. When the burglar reached to the top of the stairs the old vet shot him. The burglar later died of his wounds. Mr. Jones is lucky he lives in Kentucky. In [formerly] Great Britain he'd be facing murder charges.