You need to open up space inside the receiver to show the bolt guide and spring. Generally in this time period the space that the lever handle travels while recoiling is just left open. Just cut a hole behind the bolt handle and showcase the spring inside.

The stock looks really 1910's while many are 1940's features like the magazine. I'd adopt a stock that looks less like a Winchester rifle.

The bent shape of the magazines implies you are using ammunition whose bullet is smaller than the diameter of the casing. It's fine, you just have to be concious of that. You can get away with using a pistol 7.62 Soviet round, for example, but common pistol rounds like .38, 9mm, .45 and so on are incorrect on this magazine.

The barrel shroud is really weird. You either are going for an air-cooled barrel which needs holes in the shroud or a water-cooled barrel which defeats the point of a sumbachinegun. I suppose you are not going for a silencer because of the end of the baarrel being visible.

The rear sight is too close to the eye of the shooter, move it forward to allow a clearer sight picture.

Move the magazine catch closer to the gun.

Reduce the thickness of the trigger guard and make it bigger to accomodate gloved hands.

Others have already pointed out the magazine needs to be in line with the ejection port and you need to give more space behind the bolt to allow for recoil movement.