Ever since I was 12 and took my hunter safety course I've heard that dirty guns can backfire and that a spider web can also cause that. Is there anything documented of that happeneing?

What do you mean by backfire? Guns don't backfire, cars do.

yes I would probably have some ideas If you would describe "backfire".

About the closest thing to a backfire would be if you pierced a primer, or had a very loose primer pocket.

Then bad things can happen.

The appropriate word is "failure". Cartridge cases can fail, primers can rupture or primer pockets leak hot gas due to high pressure, or the structure of the weapon itself can fail.
We generally call that a "catastrophic failure".

None of these things can be caused by a minor bore obstruction like some dirt or a spider web.

Even serious bore obstructions in shotguns normally will only cause a bulge or perhaps a split.

they tell you alot of things in the hunters safety course so scare you to be really cautious and safe

[quote]"catastrophic failure".[/quote]

I've seen a few of these.

One of them was a bolt action hunting rifle that a guy brought into the gunshop where I worked.

The barrel was split vertically from the muzzle clear back to the receiver ring, the bottom metal floorplate knocked off,the stock split, and the bolt could not be lifted.

Now, many have not believed me, but the cause was a crayon that the owners kid had broken off in the muzzle while the gun was leaning in the corner.

The owner did not check to see if the bore was clear before firing it at the range.

You could still see crayon residue in the two halves of the muzzle, the ends of which were now several inches apart.

About this time, we got a fax from Pacific Tool Steel in Portland to the effect that they were getting reports of "catastrophic failures" of gun barrels made out of 1144 Stressproof steel, and that gunmakers should immediately stop using that steel for barrels.

Some of our custom parts were made out of Stressproof, so we cut into a piece a little ways, then pounded a cold chisel into the cut. The steel split, and the grain structure looked exactly like that of the barrel.

My boss had just finished a custom barrel from a blank, milled tapered octagonal, with integral sight bases, recoil lug, quarter rib, and sling swivel base.

He was sure that it was stressproof, and tossed that $1,000+ barrel into the scrap barrel.

His father, a gunsmith and also a hunter ed. instructor, used that rifle as a demo for years.

Another gun that came into the shop was a Colt .45 L.C. Anaconda that had the top strap completely missing, along with the three top chambers of the cylinder.

The owner swore that he was using factory ammunition, and brought along the box with 6 cartridges out of it.

The cylinder was unable to be opened, but from what we colud see of the case heads that were in the other chambers, they appeared to bee the same as what were in the box.

He wanted a replacement.

Colt said that it was an ammo problem, and the ammo maker said that it was a problem with the gun.

I can't remember how it all worked out.

But a spider web, nah, not going to happen.

I tried to find the video on the Guns & Ammo TV site last night, but they must not have it up.

Anyway, a few weeks ago they had a segment where they "blew" a Thompson/Center break-action rifle with a charge of bullseye pistol powder in the case. (30-06, as I recall)
Quite spectacular... The entire top of the barrel at the chamber came off, and the action was pretty well shattered. the forearm part of the stock was broken.
Had someone been holding it, they likely would have had a severly-messed up hand.