Well the secret is out, it was the remains of the R22 Vulcan, fitted with the latest look and fire 30000 rounds per min Titanium multi barrel gatling gun,( it does however have the larger more powerfull engine) they only have a problem with it when they wish to fire Du rounds through it, it alters, ever so slightly the C/B, this is thought to have befallen the one you saw!
Remember walls have ears, tell no one!