Defense:
Ah, yes, generally the most boring part for most people, for often the enemy will never come. However, when they do, a poorly organized and unaware force is easily overtaken and a valuable objective is lost.
When setting up a defensive position keep several things in mind:
1. Your first line should never be your last line.
When defending an area, there are generally several areas in which to take cover. Maybe some trees up front, with foxholes further in, or whatever the case is. Now it may be tempting to start your defense at the fox holes, but this is a poor idea. Start behind the trees. They do not offer as much cover but if you have to, you can fall back to the fox holes. Starting out at the fox holes or other central defense means the OpFor has a smaller area to gain control of.
Example A. My team was to assault a hill. The hill was across a swamp. We arrived and the only defense preventing us from crossing were two guys at the bridge. Myself and a few others easily slipped through the swamp and were able to take the bridge guards out, due to the rest of their perimeter being defenseless. No one shot at us from the top of the hill, either. When we scouted the top we noticed that the entire remaining OpFor element was centralized at a small sand bag area. They were quickly overrun, because their defensive perimeter was maybe 20 feet.
Had the OpFor put up more perimeter guards, my unit would of never gotten across (and fighting up a hill would have been a meat grinder).
Example B. A team was defending an area flanked on 3 sides by a deep ditch, with a centralized defensive area on top. The team started out at the edges and used the cover there to prevent the opfor from taking their own cover. Same place, different game, the opfor was able to slip into the ditch and wreck havoc on a superior force, using a defensive area that the other team should have used as a perimeter defense. Instead, they were centralized at the top.
2. 360 Coverage.
Pretty much obvious. I feel no example is needed. An open flank is a front waiting to be opened. Ensure you have guns keeping watch all around, no matter how dull things are getting.
3.Defensive Ambushes
When engaged in a siege, the assaulting team is rarely aware of their rear flank. A small unit in concealment on the exterior of your perimeter can be helpful. A quick reaction force that can flank an enemy when they begin the assault. Pinning them from the front and rear, breaking offensive capability.
Example A. My team was to guard a hill flanked on all sides by a swamp. Being an attacker at heart, I suggested I take my guys and go across the swamp to squeeze the enemy from the most logical attacking angle. A sound plan that the CO approved of. We moved across the bridge and towards the likely attack area. I spotted the opfor walking right into us and into the bridge crossing which is always a stalemate. Surprised they choose such a direct route we fell back and waited at the bridge, expecting it to be clear they waltzed in. We opened up. Too soon however. They were stopped, but only two were hit and they were mediced and pulled out. They never made it to the hill. That brings up another point.
4. Know when to fire
If your prone behind a tree watching an area, do not fire the moment an enemy is spotted, instead, wait for him and others to draw closer so a burst can take out several attackers. In my last example their assault force escaped due to eager trigger fingers.
5. Use of SAWs.
SAWs are excellent for both defense and offense. In defense, never cluster them. Try and put one at each cardinal direction. If you don't have that many, keep them ready to redeploy to wherever an assault is. But never cluster them so in the event a position is over run, not all your ROF is lost.
6. Hold at all costs.
Never give up. If you're the last man, make the place as hard to take as possible. Making it seem like there are others. Reinforcements might show up, someone might re-spawn. The objective isn't lost until your whole team is dead.
Example A. My team was guarding a pipeline. I was hit, and someone was behind the same tree as me. His gun ran out of ammo. So I gave him my rifle, when that ran out, (actually he had trouble pumping it so it never ran out) I gave him my pistol. He was eventually hit, but he keep up that one side with a pistol and 60 rounds for a good 5 minutes. Sometimes you don't even have to fire, just point and move the gun. With so many people firing the enemy won't know the difference.
Example B. Same game. I was mediced. I was one of two people left. I got up, ran, and started pumping off rounds from behind a tree. Help never came but we stood a better chance getting it while keeping up the fight then by giving up.
Offense
Advance, advance, advance.
Often times an offense is a cluster of people spraying a location from far away. These become long and drawn out, with no clear victor in the end, unless the assaulting team moves to take some ground.
Example A. We were assaulting across a wide open field with enemies in the tree line. An AK with a drum was raining death all around us. Many people held back hoping the 3000 rounds would run out soon. Myself and a few others belly crawled closer, popping up and firing a few rounds as we went. This offensive spear was only 4 people, because the others didn't advance forward. We were all cut down, but the enemy was falling back.
Example B. The lovely hill and assaulting it! Generally fire is just shot back and fourth for hours. 3 times I have crossed to lead a small unit across. 2 times it failed, because the support was still plinking across the bridge. An offense needs to be just that, offensive.
Example C. Picket's Charge. On the final day of Gettysburg the Confederates charged across over a mile of open ground. Sounds stupid, right? Its not, it works. better then it did for them. Charging needs to be done en masse so the enemy doesn't expect it, (which they never do) and so as much ammo is keeping them down as possible. Suppress the enemy right before a charge and then run. Do not fire from the hip. Keep your rifle shouldered, and burst to keep the opfor down so that your team gains land. I have done a few of them. Most are last ditch efforts with favorable results. You're usually cut down to to lack of fire power in the charge.
The 3-5 second rush. A military tactic that involves rushing from one point of cover to another in a time so short the enemy cannot draw a bead on you. Highly effective for advancing.
2. Using the enemies cover
Using the defense positions the enemy neglected to use.
Example A. My team dropped into a ditch line to assault a hill. The enemy had failed to occupy the ditch, and the area directly above it. They were all grouped at the top so a burst could take out several. The ditch assault kept the 5:1 force pinned down. It failed when 3 of us were eventually cut down and the 4th ran out of ammo, due to the sheer amount of fire the much larger force was putting down at us.
3. Flanks
New players and even the salty guys tend to ignore one of their flanks from time to time. An unguarded flank is the worst enemy to any defense. Even one player can use an unguarded flank to deadly effect. So find a flank, and keep the enemy thinking your assault is coming from else where.
Example A. It was a "training" game. My team was myself and my brother, and 3 kids with AK's. We had to assault up and take a hill. I spread out the AK kids and told them to just keep firing no matter what, even if they aren't ranging the guys. Myself and my brother moved along the right to get to their rear flank. My brother took fire and fell behind. I came up on the rear, undetected due to using the rifle fire to cover my sounds. I went prone behind a tree and was able to pick off a large portion of the enemy force.
Example B. Assaulting the same hill. My team took the LONG way to come up on their far rear. While there we were moving we were able to ambush a small unit, who was not expecting contact from the rear. The assault failed because communications errors caused the other team to go the wrong way and announce our presence.
4. Assault Formation
When assaulting try to do so in a line. It covers a larger area, and is effective for advancing forward. Be sure to keep two guys behind for medics, and to watch the rear so you're not flanked.