Jamming occurs mainly in auto or semi automatic rifles and hand guns. With a automatic the #1 primary reason for jamming is a dirty action. In order for an automatic to work several things have to happen. With a magazine type auto, first a person must put a clip into the weapon usually this action will be open and once the clip is inserted the action pushes forward pushing the bullit into the barrell and the bullit is properly seated. If the safty off the weapon is ready to begin its cycle. The bullit is fired, the spring behind the action is to open once a certain spring pressure is reached. Once this happens after the first shot the action goes back toward the stock thus ejecting the used cartrdge and moving forward again to cycle another bullit. This happens until the clip needs to be changed, upon this happening the action will remain open until a new bullit is readied for the next cycle or the ammunition is spent. As you can see many different things are going on in order for the semi or fully automatic weapons are to be cycled. The many fast moving parts and springs must be religious cleaned after every use. For most automatic rifles jam is because when they get dirty the action tries to eject a shell, the shell gets stuck on the ejector piece and at the same mili second the spring in the magazine pushes up another bullit causing one empty case and a new bullit that is trying to reload gets stuck denting or bending the fresh shell. What people don't take into perspective of what is really going on in a auto matic loading and ejocting. If you have bought an automatic rifle, you are in essence buying a very touchy piece of epuipment, and the only one I have never seen jam was a ak-47 I was out fishing in haines alaska and the owner of the boat pulled out this old rusted piece of something primordial for one there was a green ooze coming out of the magazine, my friend did'nt put new bullits in it he just fired it at some things in the ocean not killine any thing he was scaring them, the number one reason for fouling autos is lack of cleanliness

Fouling of the chamber/barrel with dirt, rust, grease, etc. Nicks/burrs on imperfect casings and dust/dirt on greased casings. Overheating of barrel/reciever can cause a shell to jam in the barrel, or the barrel to bend, stopping a bullet in the barrel.