system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If World War II era guns were so good, why did we replace them? What advancements have modern firearms made that warrants replacing standard issue weapons?
gpt
After WWII there was a pretty strong effort to look at how small unit engagements had happened and to figure out better weapons and so on from that information.  Two big things that became apparent was that it was very rare for small arms to matter much beyond around 300 yards, and that the big factor in small unit success was the ability to throw a lot of lead at the enemy quickly.

With these lessons in mind the decision was made to go from "full power" rifle rounds like the 30.06 to "intermediate" rounds like 5.56.  The 5.56 round that the M16 fires is not nearly as powerful as the 30.06 that the WWII era Garand fired, but the M16 had 30 rounds of ammo in a mag vs. the M1's 8.  Less power also means less recoil and so the soldier with the M16 can fire faster without the recoil dicking up his shots.  The M16's bullets also fly faster and flatter, so the modern soldier is much more likely to make an effective "snap shot" than his WWII era counterpart.

You also have various ergonomic improvements, a chrome lined bore, easier manufacturing, all NATO rifles were set up to use the same ammo and magazines so if you were next to a German unit you could borrow their shit if you needed it.