Taking a look at the 3D models for the MG34 and MG42, I noticed that while the textures for the two guns are different, they both share the same 3D model for the receiver. In addition, the shape of the trigger is also wrong for the MG34.
First off, here is a picture of the MG34 and MG42 side-by-side. From this angle, one can see the MG34's receiver is thinner, among other things. Note as well the shape of the MG34 trigger, which has two grooves in it to allow it to be fired in either semi-auto or full auto mode.
Now to the in-game shots. Although I took these in CMFI, the problem exists in both CBMN and CMFI. It's readily apparent that while the MG34 has a different texture, it's using the 3D model of the receiver of the MG42. From better angles, one can also see that the MG34 trigger is also modeled wrong: