Knocking magazines against things

[MoviePropMaster2008]

Originally Posted by Nyles

You don't. The visual inspection is mostly to make sure you're not loading an empty mag.

Kinda unlikely to anyone who has ever handled a magazine. Without looking, I can immediately tell the difference between a full and empty mag. in fact I can tell the difference in weight when I have only ONE round, but then I handle magazines all the time. It's probable that the soldier is checking to see how many rounds he has left.
BTW, as an armorer, we always do a visual for safety reasons, but I can also tell the difference in weight between live rounds and blanks, just by the weight. It comes with handling them all the time.

As for the original post, no one was ever formally trained to seat the rounds towards the back of the magazine by tapping on your helmet, but it was common practice in Vietnam in the field and many soldiers in the field do it quickly to make sure that you don't jam your weapon. I've loaded M16 magazines where the rounds weren't seated all the way back (loaded by hand in the field, NOT factory loaded) and sure enough, it jammed the weapon.

Before firing a weapon I always make sure the rounds are seated towards the back of the magazine.

As for the AK mags having that rib (not all of them do ya know), no soldier (American OR Russian) ever tapped the mag on his head unless he was wearing a helmet.