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I should probably amend a lot of what I just said. I'm still tracking down information, and don't exactly have the funds to go purchasing a new firearm on hand. It wouldn't be a good idea to go experimenting even if I did.

Now bolt-action, and probably semi-automatics and automatics in general simply can't fire if the chamber is open at all. They're designed to set the round properly as the chamber is closed completely and only then. If not, trigger gets squeezed and you hear nothing but a dull "click". I would assume the same is true for say a pump action rifle or shotgun. If the round isn't chambered properly, the primer can't be triggered.

The best one could hope for with a plugged barrel is to destroy the barrel and ruin their weapon. Self-injury is more than likely.

Out of battery discharges are going to give you some burns at the least. If the firearm is in good working condition that should never be a concern though. If any hot gas is coming back at the user even in a double barrel, it's likely due to sabotage, or crappy homemade design. (Lousy CSI:Miami...)

Update:
Myths
-Fuel tank bomb myth.
Hazards
-Blocked barrel

Well, the artistic liberties are what makes things entertaining. It's just that at the same time, they warp our perception of reality with our impressionable minds and such. It's kinda funny. Those heavy shooter players are probably the worst kind of people for ground warfare.

I forgot about apparel.
- Flak Jackets (Balistic Nylon and steel plates) were good for resisting shrapnel, low velocity projectiles, bites, and prickly things. I think they could even stop some handgun rounds at moderate ranges.
- Woven Kevlar is great for slowing down higher velocity rounds, but by itself it's not good for much else like explosives or knives. I think it's used with combination with ceramic plates in modern tactical vests.
- The applications of camouflage are fairly obvious. It's just better to wear as much of it as possible so as to be less eye catching. There's even gun camouflage so your metal firearm isn't creating a reflection to make you stand out.
- Helmets are handy for some protection. A helmet can make a shot that might have winged your head more survivable. Anything that restricts your joints, or obstructs your senses is probably a hazard.

Though in the event a spent casing does touch you, a scrap of clothing is much better than just your bare skin. I've been intentionally burnt by a curling iron, I imagine that's probably how bad it would be. Probably not as bad as getting branded though.

Guns are usually designed to eject spent casings away from the user anyway, but with little precision and only if held properly. This is another reason you don't want to hold a semi-auto or auto handgun sideways. If you're right handed, the casing goes upward and might hit you in the face. If you're a lefty, watch your feet. If you're wearing flip-flops you should hang your head in shame.