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Source: International Military Antiques website
This image shows a barrel muzzle clamp for the M61A1 Vulcan which was in an externally mounted gun pod used on a F-4 Phantom II fighter jet (since the early F-4s didn't have an internally mounted gun). In addition to acting as a barrel clamp, this particular design also serves as both a flash hider and a muzzle brake. These gun pods could be mounted under the wings or on the underside of the fuselage (or both). When you're mounting them under the wing, there's a chance that they'll be visible from the c**kpit when firing so flash might be an issue.
An image of the pod can be found on Flikr: F4 Phantom Vietnam War fighter jet undercarriage gatling gun pod - Mr Freek
The later F-4E Phantom models had a nose mounted M61 which featured a simpler type of barrel clamp without the flash hider.