While you are firing your gun, press on the pause button. Then, press resume. Your gun will automatic fire. The suckish thing is that you still use ammo. Try it. I have to say but maybe it will take some time or it will not happen so fire happy!!!
Always keep an eye on your radar to locate the enemy. The radar is in the upper right corner of the screen and it will show you exactly where the shots are coming from and how close the enemy is from you. Using the radar will minimise your chances of running into open fire.
If you can afford the $0.99 cost get the 'Next' weapons which are better and more powerful and will make firefights easier. When in a firefight try to hang back instead of running straight in and taking the chance of getting gunned down but if your team mates have the goat in Sacred Goat mode then stick close to them and give up your own life to save it if need be. Remember to keep an eye on the ammo in your weapons and reload only when you are somewhere that is relatively safe.