If there are already to many of these threads, then just delete it. Please, don't flame?
Anyway, I have yet to see anyone mention the utter most basic tip;
Know
Your
Map!
When you're pinned downed by multiple foes, a quick escape is much preferred to getting a kill over dying, or suicide by explosion. Being able to hop around undetected by means of a "hidden" path is a great help, too. As the old saying goes, you can't hit what you can't see.
Take some time and go through the maps with a friend, exploring alternate and less traveled paths.
This next tip, if you want to call it that, should be common sense. If you're going to camp, don't stay in one spot too often. Move around, staying in one spot for no more than 2-5 kills. Even if kill cam is off, there's still a chance that your opponent will see you after he dies. Trust me, it happens. When you're crouched down in a corner, or some such, always be aware of your surroundings. Check all possible places you could be ambushed from periodically, and be ready for confrontation.
Pick a weapon not based on its stats, but by what you like best. I find that, although it says the AUG A1 has great accuracy, that it can be less reliable. Some people are good with it, but I personally, am not. I suggest making 1 or 2 test accounts, just to get a feel for the various weapons. Try 'em out, and use the one you like best.
When modifying a weapon, I personally find that the S1 suppressor is a huge let down. It doesn't decrease recoil, just disperses it to the side, greatly throwing off your accuracy. To counter said recoil, shoot in short bursts of no more than five at long to medium range, and at close range, try pulling the cursor down as you shoot.
That's all for now, I'll probably edit later.
If you have something you'd like me to add, or something I should correct, then send me a Pm telling me so. And once again, please don't flame.
Happy hunting!
