There was another thread where it was decided that the Brit (stood next to a US Airborne fellow) was a RMP CP chap, he had an M4, so I'd say you're probably on ball. The telescopic seems to fit. The general census that they could have been issued the weapon from the Americans as (with this picture) they were guarding a higer rank, and seconded to American command.
(edited to add...perhaps a S&W Model M&P15A Rifle?)
I think it is mounted beneath the nose, not moveable as with an attack helicopter, so I guess he follows his nose so as to speak. If you have ever seen one fly they trog along and look like they should drop out of the sky...
The HK53 went out last year as due to it no longer being in production spares were getting harder to obtain and the amount of attachments that could be fitted to it such as grenade launchers, were restricted.