weapons for use against pirates...
I know a lot of navy ships have arms rooms containing M16s and even some M14s for popping mines, M9 handguns, various shotguns, and m60 or M240 machineguns, which come with pintel mount which can be popped into any number of holes for them that are all over the edges of navy ships.
Still, an M240 on a rolling ship vs a little pirate craft...I'd want a bit more range.
Of course there is the classic Ma Duce and I would think she'd be an awesome girl for the job, but history tells me that in light naval craft engangements, the ma duce can fall short. Sure, you can souse any crew, but crippling a fleeing ship is something you need to be almost on top of them to do with the Ma Duce.
This brings us to the Oerlikon 20x70 mm cannon. This fine lady was designed to take on light naval craft as well as early airplane attacks. The US Navy looked at them and passed, but later models were improved and we bought a BUNCH of them. These later guns were specifically designed to be easy to operate by non-specialists...basically by fishermen and merchant marines. A lot of civiilain craft were fitted with them. This is an idea whose time has returned. Has a 60 round drum magazine.
After WW2 the supplies and spares of this gun were running low so the Navy modified the 20mm cannon most of our aircraft were carrying. Of course this was 20x110mm so there was an improvment in range there, as well as the gun was belt-fed so you had a lot more capacity. Or more precisely it feeds rounds dumped into it, but these rounds are fed into it most often by a belt feeder and a delinker.
This to me seems an ideal pirate protection. These guns are often found on our coast guard vessles. I figure the same properties for taking on the small ships drug runners use would make it work just fine against pirates too. I also believe this was used on a lot of the patrol boats during the Vietnam war.
A while later, the US Navy stepped up to a Chaingun. Supposely this was during the Persian Gulf. It is 25mmx137 and is termed 'chain gun' because the bolt is chain driven...which means it is not a traditional automatic, but I suspect more akin to the old gatling gun if you hooked up a small motor and belt to turn the handle for you. These guns didn't fare to well on a big bouncing ship vs small ships, so they were given all sorts of fancy electronics to take care of that. This would be good for our own navy, but I suspect it would be cost prohibitive to start sticking these on the fore and aft of every merchant shipper out there.