Originally Posted by sublynx

Are you referring to the M42 gun or the MG42 machinegun?

Anyhow I haven't seen MG42's on U-boat photos. I have seen MG34's and Italian 13 mm Breda machineguns, so I wouldn't think it would be impossible that someone had used MG42's as well.

I don't think it's too likely or common though. By 1942-1943 when MG42's were introduced it must have been pretty clear to the U-boat command that a 7.92 mm machinegun is not going to be enough for air defense, and surface attacking possibilities for the U-boats were becoming very limited.