It’s time to get a little bit technical in Today’s Photo Of The Day.
You are looking at the Heckler & Koch‘s roller-delayed blow-back systems for the 7.62×51 G3, 9×19 MP5 and the 9×19 P9S.
At the bottom you have the bolt from a .308 G3, in the middle one from a 9×19 mm MP5. The smallest one is of course from the 9 mm P9S.
From rifle in a major caliber to a sub-machine gun to a pistol.