Relatively new weapon at the time, shorter range than M-14, but more payload.
Advantages of M-16
- Higher payload
Advantages of M-14
- Higher Range
- More stopping power
In real life the first made M-16s were not that good. They jammed frequently and only had a 20 round clip. The newer versions of it didn't jam as much and had a 30 round clip. They replaced the M-14 as the standard rifle for all infantry, due to it's lower cost to mass produce.
M-16 is still the U.S. standard rifle today, although it has had mass improvements since Vietnam use. And some parts were made of plastic.