Sorry to say, that while you have all of the basic elements, you don't have a proper understanding of how they fit together, or the proper proportions for things. Hopefully this will help.
What you have, is essentially an M4A1 with an M203 Grenade Launcher. The lower handguard missing - GL attachment is fine. That is how they work. One not really problem, but quirk, is that you are using an M16A1 upper receiver (with the built in carry handle) Where as M4A1's have a flat top - railed receiver, with an optional bolt on carry handle. You can see these in the example pictures below. (and the final image where the handle has been replaced by an ACOG scope) While the M16A1 receiver COULD be installed in such a gun, it's uncommon at the very least, and actually rather unlikely someone would do so.
1) The front end of your receiver area is off. The end of the upper area (either rail, or carry handle) falls in line with the end of the lower section. The delta ring (the part that holds the handguards in place) should be larger, and have only a small gap between it and the receiver.
2) Your whole front end is stretched way out. This is most noticable in your handguards. You need to pick if you're modeling an M16 (long front end) or an M4 (short front end). The examples above show an M4A1.
3) The Front sight is simply too skinny, and kind of off. You can easily find better shots of just the front sight. (Search gun part stores for AR-15 Front Sight)
4) As Racer mentioned, your trigger housing area is too small
5) You did a good job modeling the older style stock (which is still used, so no problem there) but you seem to have modeled it as one piece. In reality, it's a plastic stock, that slides over a metal stock pipe so that it can be adjusted to fit multiple people. This is apparent in the area I circled. There doesn't HAVE to be a gap there (as you can close the stock completely) but you should model these as separate pieces.
6) Racer also mentioned the odd grip. You should beef it up, and bring it back a bit.
7) Your forward assist is very small, and moved too far to the rear.
Racer also mentioned the ejection port issue. Real M16/M4/AR15's have a hole cut into the metal upper receiver. They then have an ejection port cover pinned into position, such that a spring wants to pull it open (Down as you can see in my ref image). You can close it, but it is still a separate piece. You also want to check out the topology in this area, as it doesn't match the tubular nature of that area of the receiver. (do some searches for AR-15 Upper Receiver)
9) As mentioned before your front end is too long in general, but you also need to have this little notch in the barrel if you want to mount the M203 the way you have it.
Finally, the last image in the sequence is of a more typical configuration for a gun of this sort. Take note especially, of the proportions of the 203 in relation to the front end of the gun. The mounting point on the barrel is about 1-1.5" past the end of the front sight, and the end of the 203's barrel is just before the end of the M4's barrel, and is only 3" or so past the barrel mount point. Also look at the back end of the 203's trigger area, this is meant to mount closer to the receiver of the M4. This M4 also has an RIS system in place of the regular handguards.
Good start, straighten it out, and it'll be tits!
Edit: Also, check out this page (hell, the whole forum) for far more info than you need, but lots of great close up pictures of the fiddly bits.