If you want your gun to shoot nicely and last a long time, you need to take care of it. And by taking care of it, I don’t mean just squeegeeing out broken paint.
Learn how to take your gun apart. Read your instruction manual, since every paintball gun is different. Most mechanical guns will have a quick-strip pin that you have to remove. Then you can take the inner parts out and clean them (hammer, bolt, springs, etc). When learning how to take it apart, try to lay everything out in front of you in the order that it came out of your gun. Take one part off and then put it back on, then take it off again and go to the next part. If you take everything apart at once you might be to confused to put it all back together your first time. When putting it all back together, you might have to pull the trigger while you push the hammer and bolt back in.
Clean your gun after every day of playing. When you break a ball, you can just use a squeegee to get clean up most of the paint, but after the game(s) you need to take your gun apart and clean everything. Little splatters of paint can get onto other parts besides the barrel. Even if you don’t break any balls in your games (most beginners will), you still need to clean everything. Dirt and oil from the paintballs can get into the internal parts.
Take off the barrel and pull out the hammer and bolt, and clean all those parts. I just get a paper towel wet and wipe everything down. Lube the clean parts (see below) and put everything back together.
Don’t forget to clean your mask right away. Once I got shot in the mask during the last game of the day, and I forgot to clean it off until the next day. The paint was super hard to get off, and it left my goggles tinted.
Lube it up. To keep all your parts moving smoothly, you need to oil them. You really only need to oil the O-rings, but you can also oil the hammer and bolt if you want. I like to oil them after I get done cleaning everything, but you can also just put a few drop of oil into the ASA and dry fire it a few times to oil up the inside. Remember to oil your air tank’s O-ring so it doesn’t dry up and leak or break. This is a big deal with CO2 tanks, because when you take the tank off and the excess CO2 rushes out, it is really hard on the O-ring.
Do not use any kind of oil you find! It needs to be paintball oil, or at least a non-petroleum based oil. If you use a petroleum based oil it will swell your O-rings up.
Recharge your batteries. If you have an electric gun or hopper, you need to remember to recharge the batteries. I like to take an extra with me, just in case. If you’re just playing for fun, you can wait until the battery dies and swap in a fresh one for the next game. This way you won’t overcharge your battery, and you will get more charges out of it. If you want to charge your batteries after each day of play, you can but you will get a little less life out of them.
And yes, you can get away with skipping most of this maintenance (at least with lower-end guns) but the better you treat your gun, the better it will treat you as you can read from getpaintballgun.com.