The G&G Full Metal GR-16 A3 Carbine (M4-TC) is an awesome gun.
An 8.4v 3600 mah battery (this is what I use) fits easily inside of the butt stock and it could easily accommodate a 4200 mah battery. The overall build is great, and it was definitely worth the extra bucks. Some of the pieces that normally perform a function such as opening up the dust cover to reveal the hop up do not do so on this model. For example, you must pry the dust cover open with your hands and access the hop up that way instead of just pulling the charging handle like on most models. While slightly annoying, it does mean that your gun has fewer moving parts which equates to greater reliability. The 450 rd. magazine fed extremely well and shot virtually all my bbs after winding it completely. I liked the mag so much, that I bought two more. However, it does wobble quite a bit in the mag well, so I'd put some layers of electrical tape around it if that annoys you.
The fps, rof, and accuracy were all spectacular. The handguard moves a tiny bit, but the barrel was rock solid and that was my main concern.
Overall, an awesome gun. 10/10
Great fps (I don't own a chronograph, but it is most likely in the mid-300s range....so about 340-360)
Great accuracy (especially with high grade .25g)
Can hold a 3600 mah battery easily
Hop up works really well for the guns performance
Few moving parts on the exterior, so less things to break if you drop it on the ground.
Barrel was rock solid in place
Some parts are annoying to access such as the hop up
Handguard wobbled ever so slightly
Mags wobble quite a bit unless you wrap them with a little electric tape.
Rating: [5 of 5 Stars!]