Aside from the fact that paintball guns can be physically damaged by the environment, they can also be affected in terms of how they shoot by certain usage conditions. Any time you're using mechanical equipment in the outdoors, you have to take into account the effects that the conditions will have on the device. Because paintball guns are used for a sport that can get physically pretty rugged, it's important that you adhere to a regular maintenance schedule and that you always keep the gun clean to get the longest and most reliable usage out of it.
Temperature can affect paintball guns in several different ways. If you happen to be using CO2 propellant, cold weather can have significant effects on how reliable your shots are and how powerful your shots are. This is one of the reasons that compressed air is much more popular as propellant among avid players. If it's cold outside, you also have to make sure to keep an eye out for condensation on the metal parts of your gun. When you take apart your paintball gun at the end of the day, be sure to dry off all the parts. Water is one of the most corrosive substances to metal parts and even to soft parts, such as your O-rings.
Where the soft parts of the gun, such as the aforementioned O-rings, are concerned, temperature can make a big difference. Very cold temperatures can cause these parts to crack. If you play on cold days early and late in the season, be sure to check the O-rings on your paintball gun frequently to make sure that you're not having any leakage.
It's easy to forget that, when you run, your muscles and bones are absorbing a tremendous amount of shock and distributing it in a way where it's not obvious exactly how hard you're hitting the ground. Anything you're carrying, however, will be getting shaken around quite a bit and this includes your paintball gun. Make sure you keep a toolkit with you when you're playing so that you can make emergency adjustments to your gun if something goes wrong with it. If you have a game that involves a lot of rolling, diving and other physically demanding activities, be sure to double check your gun when you're done for any dents or other damage that you may have done to it. A good paintball gun, in most cases, can hold up to quite a bit of abuse.