Knocking magazines against things
 

I just watched Black Hawk Down again, and on at least one occasion when one of the Rangers went to reload, he took a fresh magazine out and tapped it on his helmet a couple of times before inserting it. I know I've seen this before in other movies. Why do they do this?

I think it's to make sure the rounds are in line to prevent misfeeding

Tapping a mag on ones helmet does not seem like it would align the rounds at all, but I'm no expert myself. From what I gather, it just looks cool, so they do it. Doesn't seem to be any other reason than that.

Sometimes the first couple of rounds in the mag end up stuck forward with the bullets in the way of the lip, it can cause feeding problems.

That said, the way I was trained, you pull the mag out of your vest, visually inspect it, and insert it into the rifle. No banging.

How do you inspect it if it's dark?

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

Yeah, it's just setting the bullets so they are fitted back properly to the rear of the magazine so they feed right.

You can either smack it on your hand or on your helmet. Don't tap it on your head without a helmet though, it's a dumb and painful thing to do.

I wonder is the sadistic bastard who invented the AK magazine put that sharp crease on the back just to make sure people cut their hands setting the mags.

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. :D

Excuse me, I AM a soldier, and I have definately handled magazines. What's unlikely to anyone who's been in the military is the army not doing something just because it doesn't make sense to any reasonable human being. 90% of weapons drills are mostly to try and eliminate the idiot factor.

Well my Marine buddies all told me they do the tapping their magazines in and out of training as well. They say you're supposed to visually check the mags before taking them out anyway, but doing the tap is supposed to come natural