Pmags should fit in M4 receivers with no problems, shaving the tabs mainly applies to 416's and L85/L86's. Bit weird that your mag doesn't fit in the receiver.
G&G's CM16 are highly regarded by most of us, as they are excellent guns. If you want a gun that feels more like a gun, get the metal varients, G&P's, metal Classic Army varients or metal King Arms varients.
I consider gas snipers to be dangerous, as the FPS from cheaper rifles is unpredictable. Spring is better overall. However, if this is your first gun, save the hassle and go for an AEG.
£180 two tone D-boys PDW, not including paypal fees nor postage!
http://forums.zeroin.co.uk/showthread.php?260344-Dboys-PDW
Normally go for around £150 posted
http://www.airshooter.co.uk/dboy-aeg-pdw-d-boy-aeg-metal-pdw.html#.VZG_xvlViko
The built in visors have massive gaps between the user and the lens, so bb's can easily pass through the gaps which is extremely dangerous. Not recommended.
CM16 for £150
http://forums.zeroin.co.uk/showthread.php?260300-g-amp-g-cm-16-new-some-accessories-too
"new, unused" yet there are visible signs of wear on the magazines and on the gun itself.
Yeh, belts or webbing are brilliant, but plate carriers get hot. I've got a Tactical Tailor 2-piece MAV and I love it, but there are many others of similar design made by WAS too.
To put it bluntly: because not many users on the forums are willing to pay £100 for a transparent/two tone gun, that's how it is. Dropping the price over time will attract buyers.
Where are you paying £17 for them? They're normally £15
http://www.ebay.co.uk/itm/DEVIL-BLASTER-DIAMOND-GRADE-0-3G-3000-WHITE-AIRSOFT-SNIPER-/121343296673?pt=LH_DefaultDomain_3&hash=item1c409fc4a1