There are lots of training techniques that can help you out off the field. Getting shot at while playing is a completely different set of training. Both are much needed.
You can train your aim by just using a gun and aiming. A mag is great for this because it is a hollow tube when the valve is not installed. When you are sitting and watching television, practice aiming your gun at objects around the room or at the television. Look through the hollow tube to verify and adjust your aim. It is very simple but it trains your muscle memory and will make you a better snap shooter. Also practice walking around with your gun up, held steady against your shoulder, and aimed. It will train you to keep your gun steady when moving. It drives me nuts when I see my players moving on the field with their guns aimed at the ground.
Think about bunker moves and situational play. Decide what you will do in any given situation. Keep going over those scenarios in your mind. When game time rolls around, you will already have the thought process worked out and you won't have to make a long calculated decision on the field. Each move will become an instantaneous reaction.
Next you need the muscle reaction. That is physical training. Doing squats and sprints are great for leg training so that you don't run out of burst speed on the field when you need it.
You also need real field experience. Practice shooting at running targets. Do this with a friend from about 100 ft. It won't hurt at that distance. Use semi auto. You don't want to aim by following the stream of paint. You want to learn to lead your target. Have your target run from bunker to bunker across a field. Then reverse the process and have him shoot at you. It is just as advantageous to get shot at as it is to shoot. If you don't want to shoot each other or get shot, you can use targets such as old hub caps. Have someone toss them across the field so that the flat side is towards the shooter and shoot at them. They make nice sized targets and are heavy enough that they don't secumb to the frisbee effect.
Except for the Automag in front, its usually the man behind the equipment that counts.