so far the only worth-money firms are Tokyo Marui and Classic Army, the rest is mostly worthless crap. so, be sure to pay attention on the manufacturer of your gun-to-be.
if you say you wanna have something like M16 or AK47, i sugest you buy an AK-47B-as in Betaspetznas, ok the gun doesn't actually exist, but it looks a lot like one of those SF moded AKs, like our special forces use, or at least used to. but the best part is, the gun is really sturdy, not too long so you can use it in CQB and again sturdy, compact, rarely do people brake it. with M16/M4 you are bound to change the body-the ears that hold the bolts brake easily and EVERYONE have to buy a new METAL body after 10 "field trips".
DON'T you use AT LEAST SUNGLASSES!!!! use eye protection that is cetified as! BUT if you are going to use JUST SOME sunglasses, fire a burst in them from one meter of course while you're not wearing them!!! don't play with your eyes, you ONLY HAVE TWO!!!! when playing CQB use teeth protection aswell, i know at least half a dozen people who have lost a tooth or a part of one!!! use it in the forest as well, but it's not that risky!
if you can't get teeth protection, don't laugh while playing (i know it's hard, hehe)
i have a P90 and a G3-SG1, both are TM, and i've said it like a hundred times, i have to gear up and take a pic of myself, but...i just don't do it