Though i havn't been around for very long I've heard a common problem with M4s and their variants is mag wobble or ever worse the misfeed, from what I gather the magazine needs to be pushed forward for it to seat properly and fire. However after a few shots the magazine would rock back towards the grip and would not feed.
I had this problem with my CA LWRC so I walked into AONW and declared that I was going to find a mag that worked. I did. However the only mags that would work were, at cheapest, $25 each. Yes I'm a cheap bastard so I bought a box of 10 pmags for $80 and vowed I would make them work.
PLEASE REVIEW THE PICTURE AT THE BOTTOM OF THIS POST BEFORE BEGINNING
Materials:
1. M4/Variant (derp).
2. Desired Magazine.
3. Epoxy Putty
4. X-Acto Blade
5. Sand Paper (Medium-fine grit)
6. Gloves (Unless you want to scrape putty off of your hands for 3 days)
<WARNING> Epoxy putty dries quickly, you have about 5 min of working time!
Fix:
Put on the rubber gloves and mix a small amount of putty. Apply directly to the forehead (NO) Apply to the back interior of the magwell (the part that is closest to the grip) Using your fingers push the putty against the magwell as smooth as you can and insert the magazine you want to use. If you are having trouble scrape some of the putty out and try again.*
Once the magazine is in, mix another small quantity of putty and apply it to the lip of the magwell so it slightly pushes the mag forward, thus achieving proper feeding. Hold the putty that is applied to the magwell lip and take the mag out. (Remember to wipe any putty off of your magazine)*
Continue to mold the putty and reinsert the magazine until the mag seats firmly and feeds correctly, a good way to do this is to load the magazine and listen for the bbs feeding up into your gun.*
Now when your magazine is properly seated without you having holding it forward, take it out of the magwell and let the putty dry COMPLETELY. You have to do all of the steps above in about 5 min. because of how quickly the putty dries.*
After the putty is dry, it's pretty much trial and error, using the sandpaper and X-Acto blade to sand and or cut any putty that is causing the magazine issues. Still not feeding correctly? Add small bits of putty until it does and remember do not leave the magazine in your gun while the epoxy is drying. Don't be too scared if you horrendously screw up, it is possible to remove all of the putty and start from square 1, it just take a lot of cutting and sanding.*
If you are still having trouble with mags not fitting, get at close as you can to a loose fit, then just slam the magazine into the magwell and eject it, repeat this until perfect fit

PM me for any questions/issues
Lastly +1 to AONW for being the best airsoft store ever! Great products, knowledgeable staff, couldn't ask for more.
P.S. Thanks for the patience and help Gunny
Clear view into the magwell, and where the putty should be:

Here you can clearly see the small lip of putty pushing the mag slightly forward:

*Disclaimer: I am not responsible for any 12 year old filling their magwell with bondo. You assume all liability by reading this and promise not to sue me for your inability to read and/or your colossal shazaam ups.