First off, I want to address the MG-13, the in-game model is that of the MG-42, and the MG-13 used a slightly curved box magazine of about ~20 rounds. The MP 28 looks alot different than the MP 40, the MP 28 having a side mounted magazine, and wooden stock. Let me provide some videos from a renowned source for exotic, and historic fire arms.
They know, that those weapons have different model. Those are only placeholders, which will be changed as soon, as they finish real models. Same was with MG34 in Berlin test. It also had MG42 model, but now it has its own.
Don’t worry! Guns that appear to have the wrong weapon names are just place holders! But we do appreciate when you find and report anything that is wrong historically:)
Hey, no problem, Im glad you guys are working on it, thanks for making a fun game. Have a good one 
Thank you for your effort in historical accuracy and to develop this great game. However, there was yet another finding, that may cause issues with historical accuracy that is about Pz III B, that is currently as a researchable option (previously very first tank at disposal) despite apparently never being deployed in operation Barbarossa. For example, this thread deals with the issue but seemed to be left without much attention. If the developer team is already aware of this, I´m sorry for pointing this out again.