Shipbucket

It used to be, but not for the reasons people think. There's no laws against carrying weapons in international waters per se, however, there's plenty of national laws against carrying and (Especially) using them in national waters.
Those skiffs are surprisingly hard to sink. Point in case, one of my friends served on the Absalon when they caught the first batch of pirates. After they'd taken the pirates into custody and confiscated all their gear they tried to sink the skiff with 12.7mm fire. One box of ammo later and it was still floating. They ended up using a scuttling charge.

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“Close” only counts with horseshoes, hand grenades, and tactical nuclear weapons.
That which does not kill me has made a grave tactical error

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Source Materiel is always welcome.

I'd go with a Mk 19 Grenade Machine Gun. That'd do the trick. And yeah, they should go with something in 7.62 for rifles just for better range. I'd have something like an M-14 or a derivative.

The United Nations Convention on the Law of the Sea grants ship-owners that they can take measures to protect themself, ship and it's cargo. If I remember max cal for self-def weapon is 12.7 mm

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Nobody expects the Imperial Inquisition!

Well, the pirates are obviously doing illegal, so why not join in and use big guns?

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"If people never did silly things nothing intelligent would ever get done." ~Ludwig Wittgenstein

Because using the big guns is illegal, expensively and NS style.

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Nobody expects the Imperial Inquisition!

LOL, not to mention a glorious amount of overkill. One 57mm round for a Bofors 57mm cannon costs like US$1200. Why spend that when you can kill everyone in an open skiff for a few bucks worth of 7.62mm or .50 cal.?

The most effective first line of defense is still going to be something like an LRAD, then a security team with sniper rifles and M240s with carbines to repel boarders. You don't need to out-gun them, just out-range them.

Exactly. And it's not like there are a lot of pirates out there who have decent training in the finer points of marksmanship.

Training, no ... but they can by a vets of civil war that rage thru Somalia from long time. Some of them know how to shoot and even hit targets. Well ... there is no problem with hiting cargo ship.

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Nobody expects the Imperial Inquisition!

That brings up another point, even the mostly highly vetted and skilled of marksmen the Somali pirates have are unlikely to use anything other than the bare (and likely highly worn) iron sights their weapons have. A guy with an M4 (and a high-quality scope) might actually outrange them by a significant margin. Even trying to fire an RPG on an oil tanker requires getting well within the range of say a 7mm mag, let alone a Barrett.