

The round thing under the eagle is a Nazi symbol. https://en.wikipedia.org/wiki/Sun_cross?wprov=sfla1
It’s also an ancient symbol for the sun, but so is the swastika.
EDIT: It looks like the whole image is the Nazi designed poster for the Olympics. Like I said in my other comment, they could have just used an image of Jesse Owens for that particular Olympics. Would have been a better message.









Open bolt guns are more often dedicated full-auto type weapons. LMGs and Sub-machine guns. This is done to help the weapon cool down and prevent ammo from cooking off in the chamber.
Typically the firing pin is fixed to the bolt and it is designed in such a way that the round doesn’t fire until the bolt is completely in battery.
The bolt remains open until the trigger is pressed. The bolt slams forward, picks up a round from the magazine, and chambers it, firing as soon as the round is chambered.
Most modern SMGs are closed bolt these days, the bolt falling with the trigger doesn’t lend itself to accuracy.
Here’s a video of gun Jesus explaining: https://youtu.be/Ga4uoWmfEt4