am starting a 1/350 emden and the six 4.1" deck guns on the model look more like turrets than open mounts with shields. the 2 sides are hollow so i plan on cutting out the back and building 6 breech assemblies. (have the 1/96 plans). i would also like to open the two 4.1" aft casemates.

does anyone know where i might find 1/350 (or 1/400) open mount surface guns (say 3" to 6")?

I don’t know of/ or have seen any manufactured (though that is not my forte).
But, NavWeps says these guns are still in use in Norway (as of 2002), so perhaps there is a Norwegian costal defense kit out there with them in the scale you would like.

350 scale is not as prolific as 700. you might pop off an email to Yankee Model Works or Iron shipwrights and see what thye may have. More than likely you are going yo have to scratch