Mostly correct. The chroming of the barrel was critical to its effectiveness. The 5.56 round of the Vietnam era is no longer in use my the military. The powder behind the round changed mutiple times in the testing phases (early 1960s) and then again once the rifle was fielded to line troops. Another major, yet rarely discussed change, is the revolution ratio of the round.
If I recall correctly the older Vietnam era round was a 1/11 ratio. This gave the round the ability to tumble once it hit something, say hitting a man in the arm yet exiting his chest and tearing up bone and flesh in the process. The theroy was that the tumble would make up for what it lacked in knock down power. The problem with this approach is that it would also tumble when it hit a small branch, or most anything. It simply lacked penetration power. Accordingly, its accuruacy in combat was greatly effected. Also, the round itself was not a dumb-dumb styple round designed to seperate on impact, only tumble.
The newer round adopted in the mid to late 1980s (Standard NATO Ball) still has the high velocity, but at 1/7 revolutions it is a maximum stabilty round. So, it does not tumble nearly as much as the Vietnam era round and it will not do as much damage. However, it is more accurate (in combat) and it has FAR greater penetration. In fact I workled with FN on upgrading the M249 SAW in 1987 and during which time we tested the 5.56 (SAW) against the 7.62 (M60) ands concluded that the 5.56 actually had better penetration on steel at 200 meters and roughly similar on wood at 200 meters. This really surprised me.
The new round when fird from a SAW can be accurate on "area" targets up to 700 meters if I recall correctly.
As many here have stated the M16 is a good weapon with a good round. 300 meters really is nothing. Most of the older M16s have been refitted with 1/7 barrels.