That's a blank firing adapter. Blank ammunition will not normally cycle the actions of self loading firearms so a blank firing adapter that constricts the muzzle opening and builds up gas pressure to an operating level is installed.

>>>Reading about the USS Cole, they had plenty of time to machine gun the the bomb-laden boat into swiss cheese, but the engagement protocols prevented them from firing without the captain or first mate's permission. Very odd.
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>>Not really. I don't know about the Navy's policies, but the Army was very strict about when ammunition was even issued, and when it was to be used. When I was on patrol, we had loaded magazines, but the tops were taped over. We were accountable for every round issued, and under strict orders regarding the actual chambering of ammo. Command would rather risk casualties than live-fire incidents.
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>Talking of which, what are those red things at the end of the barrel when on exercise? The British army use yellow ones. I presume they're to prevent the rifle from firing live ammo (but of course blanks can be fired) by some means, but I don't see how they work.