Aftermarket ship's guns

I'll second stik's thoughts on Shapeways.  I've gotten 20's from them for a 1/144 and 1/72 Gato and they are absolutely beautiful pieces.

Sorry i neglected the scale. Yes I am building the 1/350 ship. Thanks to everyone who replied.

can also check this link. http://www.3dmodelparts.com/1-350-single-20-mm-oerlikon-cannons-pedestal-mount-12-pcs/

http://www.3dmodelparts.com/1-350-us-navy/

Have a look at Shapeways for some 3-D printed 20mm Oerlikons. They are much nicer than PE parts, and quite well detailed. They offer them in various scales.

Well, assuming you are looking for 1/350. I've bought L'arsenal guns, it looks like there are one or two others.

Check Freetime Hobbies.

You might also check how many you need. Not that Trumpeter gives them away, but I've been saved a few times where they include a sprue, or multiples of the same one, that cover the needs of a ship in the same class, but a different AA fit.

I'm assuming you want the 1/350 singles?

https://freetimehobbies.com/1-350-infini-models-20mm-oerlikon-cannon-mk-iv-mount-set-a/

Otherwise a dedicated PE set for the Mass should have some, all be it flat, but better detail than the kit parts.

Fine Molds produce some 1/700 20mm singles and the Chicago Piano in the same kit. I don't know the quality though.