They didn't. That was something some US troops did in Vietnam, hoping to make sure the cartridges were not in a bind inside the magazine. Among many problems the M-16 rifle had was a tendency to jam, for many reasons, one of which was poor feeding of cartridges from the magazine.
The old M-1 was much more reliable.
Some other weapons in the WWII light infantry arsenal had magazine feeding problems, particularly the Thompson. Most solved this by loading only eighteen rounds in the twenty round box magazines, to avoid over-compressing the magazine spring.
Also, sometimes they did it when they want to find out if it's an empty clip or not. Nothing is worse than inserting an empty clip into a rifle in a middle of a warzone. I bet you watched it from "Saving Private Ryan".
No
In world war one, helmets where made of steel, to help protect your head from any bullets or other weapons used (:
Leather, after 1916; steel.
US Army M1 steel helmets were not issued with cloth camo covers as were the US Marines fighting in the Pacific, during WWII. Consequently, soldiers didn't do to much writing on steel. The common thing was to paint their division patch on the sides of the helmets (1st Infantry Division, 3rd ID, 4 ID, etc.). In Vietnam, the US Army began, for the first time, issuing cloth camo reversible covers for the steel M1 helmets. At this time US Soldiers began writing graffiti on their helmets.
Very little or none at all apart from steel helmets
A Kevlar helmet saves countless soldiers lives every day by being bulletproof. Kevlar helmets help soldiers stay safe from bullets, shrapnel, and explosions.
It shows what side a soldier is on from the design and even helmets today can deflect some kinds of bullets.
no
blue helmets
the shapes on the sides of helmets were used to identify a soldiers regiment the spade was (506th parachute infantry regiment)
No
They wore helmets long before world war 1.
Not all gas masks are compatible with helmets, so some soldiers forgo helmets to wear their masks.
the helmets protect them a lot
In world war one, helmets where made of steel, to help protect your head from any bullets or other weapons used (:
Leather, after 1916; steel.
The seamen and officers who wear red vests and red helmets deal with ordnance. (Bombs, bullets, missiles, depth charges, and so on). They are also the people that deal with crashes and salvages.