I found this nice and interesting... from ehow.com
Want to give yourself an advantage while playing airsoft? Well why pay for expensive upgrades or a better gun when you can take these simple steps. Just follow these instructions.
Step 1: The most important thing in airsoft is accuracy. It doesn't matter if you're shooting long and hard if you don't hit what you're aiming at. So first you'll want to buy some decent BBs. You probably paid a lot for your airsoft gun so spend the extra $2 on high polished, seamless BBs. Most automatics require high polished ones anyway otherwise they run a high risk of jamming. If they're seamless they're perfectly round with no holes or seams or anything. Being Seamless and high polished will make them not rub on your barrel as much on the way out which causes them to fly crooked and curve incorrectly. But WAY more important is that the BBs are balanced evenly on the inside. Ones that are either filled or stamped together with a seam are off balance and that can cut your accurate range in half! The high polished BBs instead of regular seamless ones just have smoother edges that will make it slide through your barrel faster with less friction to slow them down. Also as long as your gun shoots at about 240 FPS or faster, use .20 gram BBs unless otherwise specified in the manual or on the box. .12s curve too quickly in the air with very little wind.
Step 2: Now that you're firing ammo that's actually good, you may need to adjust the hop up. Usually it's adjusted well from the factory but it's worth playing with. Basically just find some cardboard or styrofoam or whatever and draw a tiny black dot on it about 1/4 inch wide. Then stand about 50 feet from it with your gun. If it's a pistol maybe move a little closer and some high powered rifles may need to fire from farther away. Put the gun in single shot mode if it's an automatic and aim directly at the dot you drew and fire a shot then wait and fire again until you've shot like 3-5 shots. Make sure you re-aim it perfectly in between shots, especially if the gun kicks back.
Step 3: If all your shots missed the dot in 3 different directions then it's likely your airsoft gun just isn’t very accurate or you’re still using low quality, unbalanced BBs. If you were away from the black dot but in the same direction like all three shots made holes above the black dot, you were either aiming incorrectly or you'll need to adjust the hop up one way or another. The best way to tell which is the case is to try to watch the BBs while they're flying on the way to the target. If they go straight, you're good. If they curve as they fly, that's what's causing the inconsistency. If you can't tell, make sure your scope or other aiming device isn’t crooked though and that you’re not making any aiming mistakes. If you know it’s the gun's hop up causing the problem though, most have a slider for hop up adjustment so just pick a direction and redo the test and if it gets closer to the dot, keep moving it until you hit right around the dot without a lot of the BBs curving way off path.
Step 4: An alternate way of adjusting your airsoft gun's hop up is to purchase some .20 gram, high polish, seamless, glow in the dark BBs. Don't get the .12's, they don't hold enough light for long enough. First wait until it’s dark outside then head to a wide open space like a field. If you have an open magazine that exposes the spring and the BBs, take out the magazine and shine a bright flashlight as close as possible up and down the row of BBs to glow them up then quickly load the magazine back into the gun and fire them level with the ground as if you were aiming at a person. Watch the glowing BBs carefully to see about how far they go before they curve and fly in a completely different direction. Adjust the hop up lower and lower until the BB starts to drop down instead of curving off in a crazy direction. If you can’t get your gun to do that, as not all will be capable of it, just adjust it until it curves as far out as possible. One of my airsoft pistols straight from the factory only got about 30 feet before curving at a severe angle. I could almost fire around a corner with the thing it was so sharp. But after adjusting it, it remained straight out to a little over 60 feet. Also, if you switch the weight of your ammo from .20g to .25g for instance, you’ll definitely have to re-adjust your hop up.
Step 5: So now that your airsoft gun is properly adjusted and firing good BBs for optimum accuracy and speed, you should probably take the one extra step and get some silicone airsoft gun lube. You just spray it down the barrel and it lubricates it with a thin layer of silicone so that passing BBs have way less friction. Technically you should do this before adjusting your hop up but it shouldn't affect it too much. Plastic sliding against not very well polished metal is a friction nightmare so you can seriously add around 30+ FPS to an airsoft gun by lubing the barrel. Do use airsoft gun lubricant, not just any hardware store silicone lubricant because it might have additives that will damage many of the internal parts. They almost all contain propane distillates that will dissolve plastic and turn it white and you don't want that happening to your gun. Once you've done this, the BBs will come out straighter and much faster because they cruised past any microscopic bumps on the inside of the barrel. One coating should last hours but do reapply it if your guns have been sitting for weeks since the last time you applied it.
Step 6: And finally, you can upgrade springs but this is risky and not always very effective. On all airsoft guns with springs, it can wear out the parts like the air chamber where the piston moves because it’s not used to a higher pressure. On manual spring guns, it can wear out other parts like the slide because of the higher force required to pull it back. On automatics it can cause a lower rate of fire and will definitely wear out the motor faster because it has to work harder to pull back the spring. In some cases the whole thing won’t be able to function with the new, harder to pull back spring. Usually companies put the highest powered spring they can in for the motor's power rating to get the highest FPS without overloading the motor so any thicker, tougher springs may break the motor. Also just taking apart some airsoft guns risks breaking certain parts of them. So in general, I wouldn’t recommend upgrading the springs unless you’re a real risk taker.