Many people claim they cannot sight their guns with a mask on. Here is a simple method that allows you to use iron sights or scopes and still keep your face protected.
Shoulder your gun and tilt your head down.
Now rotate your head so the nose section of your mask comes down to the stock of the gun.
Your head should be facing towards your gun, as if you are looking somewhere between the mag and the trigger.
Look out the corner of your eye and you should be able to see through your sights.
Sighting in a scope should be done the same way and would probably be easier because most optics are positioned higher than the iron sights.
Here are some photos of Grant holding various guns and looking through the sights with a mask:

Head down, Nose towards gun, Eyes level with sights.

As you can see my right eye is able to line up the sights on an M4

Looking through the scope on a G36K which many players claim is impossible with a mask.

Front view of sighting the G36K.

Looking through the low sights on an AK-47.

Front view of AK-47.
Not only does this method let you use your sights with a mask but as you can see in the photos it allows a smaller profile making it harder for your opponents to shoot you.
Have fun and play safe.

