For those having issues on how to aim accurately with the iron sights, see the below images:
Imagine the red dots are heads, or whatever you wish to hit. Up to 200m you should not have to adjust your sights. Place the tip of the front sight over what you wish to hit, like an arrow saying "put bullet here" and fire away.
However with some weapons, due to recoil, the sights are a bit different when it comes to hitting what you want. As an example, take a look at the PPSH:
The recoil and rate of fire causes the aiming to go all over the place, even when controlling the recoil with the mouse. Place what you want within the sight hood area shown, like a picture frame.... and then unload. This also works in the same principle as the MP40, but since the MP40 has a slower ROF and recoil, the accuracy is a bit better.... still, place what you want within the sight hood, rather than using the tip of the front sight.
The DP28 also has an odd way of aiming when it comes to getting an accurate shot off:
Again, imagine the red dot is the target you wish to hit. Use the central area noted by the front sight and the two curved spots of the sight as a centering point and fire away.
With Full Auto weapons, Burst shots are the best way to maintain your accuracy, as well as your ammo.