What, only the old guys can answer this? Well, the history of the M16 involves the Air Force, war's continuing demand to carry less to do more, and typical American internal politics.
Suffice it to say that the M16 was NOT originally designed to use Ball gunpowder, but Improved, which was a lot cleaner. The ARMY decided to use the old dirty powder, which leaves a lot of residue in the gas system, necessitating chromed rifling, chromed lined gas tube, etc. The Vietnam environment prompted a foward assist because the design weight of the bolt is light, compared to most, and has much less inertia to overcome dirt and gunk in the reciever. (In the other corner is the HK G-3 bolt, a bludgeon in its own right. The HK can digest crushed limestone and fire. The AK can, too.)
Also co-dependent in design is the 5.56 round itself, which was designed to be on the edge of the ballistic envelope. On impact the round becomes "dynamically unstable," meaning statistically more wounds requiring a larger logistic burden, directly on the battlefield, up the medical chain, to the owning government.
Today's M16s are product improved, i.e., the burden of dead people forced changes to the design. This is known as the law of unintended consequences. If the Army had used the SPECIFIED powder in the ammo, little of this would have happened. If we didn't need bases in Italy, we might not be using Berettas.
Do You Really Want An M16? Understand it's design purpose and match it's abilities to the job at hand. It is not a long distance, heavy bullet, high knockdown, fast stopping weapon. These are attributes in demand for personal defense. Is War personal defense? Read the Geneva Convention from the perspective of government liability, and the M16 fits. From the perspective of "1*", as in Gary Paul Johnson's motto (sound it out slow - er), I am better served by a good ol' 30-30. The firepower of an AK-47 with a low threat look.
Personally I'd like a hybrid - HK reciever, AK gas & bolt, FN stock, Multipurpose handguard, in 30 cal high eject. Each round loaded propels the previous cartridge as the bullet. Just double charge like any MG to load. BUT I don't need it.