M4A1 Modified (WIP) (Please Help)

Marto101

Hey there, I'm an aspiring 3D game artist and I'm looking for some help. I'm trying to do a SUB-D modelling practice of making a M4A1, I'm having a little trouble making the middle shape of it (As referenced in first picture below) and in general with some of the ideas on how to best get shapes and what to model in vs what can be done in normals. 
I would much appreciate some critiques and feedback or more on where to go with this and what I should be doing, I'm having trouble making progress on it. Below is two of the main references I am using accompanied with some screenshots of my model>
Thanks very much in advance!

ZangZing

Hey, looking good so far. The only thing I would take into consideration are edges. More chamfers - better normal map. For example:
Notice how rounded this edge is, and compare to yours (I made this mistake with my m4 also, a lot of sharp edges). Also don't worry, m4 receiver is pretty tricky to model so keep trying.
Cheers

Marto101

Made some more progress on the middle, will be doing a bit more once I get back home as I will be moving soon. please any more help is much appreciated, getting the bullet dispenser point is a pain in the ass and getting the right shapes is killing me, all help is welcome.

John Baxter

Nitpicky, but I think the raised bar shapes area under the ejection port should stick out a little bit more.

Marto101

Making progress slowly but surely, Check it out

Marto101

Done a bit more work on other parts of the gun as well, just fixing up the high poly pieces and where they sit along with going on that centre piece, it just requires a bit more work to get to point that Im ready to make it a whole model and sort out the other details. Again, all critique and feedback is appreciated