Oerlikon 20 mm cannon

HR Products makes some white metal Oerlikons in this scale.  

Be warned that they are very roughly cast.  The time and effort necessary to remove the stuff which is there and shouldn't be and to add the stuff that isn't there and should be may be better spent scratchbuilding your own. There are several online dimensioned drawings of Oerlikons to use as a reference.

I melted mine down to use as airplane nose weight

I'm weighing in on this late as I often do but the HR Products 20mm Oerlikon are okay as far as the pedestals (early style) and gun shields are concerned. The thin gun barrels, however, are too soft for my liking. The shoulder rests need to be fabricated as well and the rest of the comments on the overall quality are spot on.

If it's any consolation, the HR 40mm Bofors guns are much better or beefier I should say.