The M16A4 was the standard service rifle for the Marine Corps until October 2015, when it was decided that the M4 Carbine would replace them in infantry battalions. For whatever reason, civilians tend to think the M16A4 is awesome when, in reality, it's actually despised by a lot of Marines.
Now, the M16A4 is, by far, not the worst weapon, but it didn't exactly live up to the expectations laid out for it. They're accurate and the recoil is as soft as being hit in the shoulder with a peanut, so it certainly has its place. But when Marines spend a considerable amount of time in rainy or dusty environments, they'll find it's not the most reliable rifle.
Here are some of the major complaints Marines have about the M16A4:
1. They get rusty very easily
For a weapon that's supposed to be used in "every clime and place," these rifles seem to get rust like boots get married – way too quickly. This just means that you should carry some CLP and scrub it off regularly — another task to add to the pile.
2. Cleaning is a headache
Outside of problems with rust, the chamber gets caked with carbon after firing a single magazine. This is yet another thing you'll have to spend time cleaning. And when you break the rifle down, you're going to find carbon has found its way into every possible small space.
3. Jams are too common
If there's a bit of dirt in the chamber, prepare for some double feeds or stove-pipe jams. This might just be the fact that many of these rifles have been worn down from participating in two separate combat theaters, but the fact remains: your gun will jam.
4. They're too long
An M16A4 is nearly 40" long. For close quarters, these really aren't the best weapons. You'll have to find ways to adapt the rifle to the environment but, at the end of the day, it's a pain in the ass to try and jump through a window with it.
5. Rail covers make the hand guards slippery
You could just refrain from using covers, but without them, you run the risk of degrading your rails. With them, you won't be able to get as steady of a group, which means your per-shot accuracy will go down slightly.