After viewing some of the recommendations and class set-ups via ELITE, I decided I would share my suggestions for a few of the weapons. I know each player has their own playing style and preference, but this site isn't just viewed by the veteran players, but also the more casuals and beginners of the game. Leave a comment on what you believe benefits the weapon and its performance - or how you use the weapon and the reasons why you use it. Anyways, read ahead...
The M16A4 is unlocked at rank 4 and retains its low recoil, moderately-high damage, and fast reload time. Despite its statistics, the M16A4 suffers in close quarters combat - as all three shots must land to kill in a single burst - but excels in long range battles. Due to its versatile nature, the M16A4 can be customized with a wide range of attachments and proficiencies depending on the map, game mode, or based on one's play style.
As far as attachments and proficiency selection goes, combining the Red Dot Sight with the Rapid Fire attachment via the Attachments proficiency will ensure that you not only stay on target but also land those extra bursts incase the first few miss, or for the additional damage. An alternative recommendation: combine the Rapid Fire attachment with the Focus proficiency. This will assist you on landing your shots when taking fire when you enter those long range one-on-one firefights so you can be as accurate as possible (accounting if you're used to the M16A4's iron sights).
For a more aggressive and 'on-the-move' class, Sleight-of-Hand, Quickdraw, and Steady Aim Pro is a simple yet effective set-up as each perk benefits from the previous tier. These perks will make certain that you enter and ADS on time - and ahead of time - beating your enemy to the trigger. They will also guarantee the M16A4 will perform in close quarters, and in case you're out of ammo or need to reload, switching to your secondaries will surprise your enemy. Switching Steady Aim for Dead Silence Pro will allow you to get up close and personal without being detected by the enemies. If your cover is blown however, you can safely flee from high buildings and lookout points without taking fall damage. The AH-6 Overwatch will take down enemies you can't reach, or those sneaking from behind.
If you're looking for a defensive and tactical approach, Recon, Blast Shield, and Marksman Pro is one alternative. Assist yourself and your teammates by marking them on the UAV with Recon - combining the Impact proficiency highly benefits both the Pro variant and the M16A4's performance. You can stay near choke points without the fear of being killed from enemy grenades or explosives, and you can stay on target when flashed or stunned. Marksman will allow you to identify targets at longer ranges and catch them off-guard. Using Sitrep Pro in the place of Marksman can let you hear out the enemies nearby and spot equipment from afar so you can maneuver around the map, find enemy hideouts, and enter without taking them with the help of Blast Shield. The Trophy System can be an assistance in hotspots and near objective points. Using the Ballistic Vests, Sam Turret, and Recon Drone is recommend.
The M16A4 can be obsolete compared to the other assault rifles in its tier, but with the right set-up it can be just as deadly.