The same gun that Cyril Maude used to kill Revell was used to kill Attard.
But Maude's gun was stolen (the Irregulars who visited Maude's house will make this clear).
Maude's murderer probably stole the gun.
Curtis Twiggs is the murderer.
If you read Twiggs's autopsy, you should know that he had a third bullet wound in his shoulder from a small caliber gun.