Due to the limited number of reply words, I can only add here:
Reply: I had difficulty typing on my mobile phone when I slept that day. Please add 1. The power of the projectile is related to the weight of the projectile, but also to the state of the projectile after hitting the target. After AK47, short assault rifles, most people think that 7.62mm is the best standard for infantry. However, after studying the rotation speed of the projectile, Americans thought that the wound caused by the rolling and splitting of the warhead in the target body was more intense, so they reduced the rotation speed of the medicine outlet chamber (increased the winding distance) and reduced the caliber, and M 16 was born. Although unpopular in the Vietnam War, at least the development idea is correct. At present, weapons in all countries are transitioning to small caliber, that is, 5.56mm and 5.8mm in China (I don't know why they are not common with Europe and America).
The positioning of submachine guns is different from that of rifles. At the second stop, it is necessary to suppress the firepower in the face of melee, but the machine gun is bulky, expensive, inconvenient to carry and unable to fire quickly to form coverage. So submachine guns were born, n years earlier than assault rifles. At this time, rifles are still shoulder to shoulder, single shot, wooden support (just like brothers). Submachine guns use 9mm pistol bullets, with blunt warheads, much lower charge than rifles, and much lower range and penetration, but enough for close combat. Of course, the gun itself is simpler than the rifle, and a large number of stamping parts are used, which reduces the number of parts, caliber and charge, reduces the requirements for the barrel, and thus reduces its overall cost. However, the Thomson submachine gun in the United States seems to be not so lucky, and it is very complicated and expensive.
The official statement is that the different positioning of rifles and submachine guns leads to different calibers and ammunition. So the caliber of the bullet depends on the purpose of the gun itself. In fact, there are pistols with a caliber of 7.62mm or even smaller in history, and the bullets do have high penetration. After hitting the target, they can be transmitted from the rear, with little trauma and even no loss of combat effectiveness.