what would make an sks go full auto? lets say someone was shooting winchester brass cased ammo in a chinese sks and it just dumps all the rounds with one trigger pull and the last round didnt fire the last round but the primer had been struck.  whats wrong with the gun?

The SKS is designed for hard military primers and it is notorious for slam firing with soft primers in brass cased US commercial ammo.  Stick to steel cased ammo to alleviate this problem.

The other factor could be garbage around the firing pin inside the bolt that restricts the movement of the firing pin inside the bolt.  This is especially common f its a new rifle and there is cosmoline still surrounding the firing pin.  This is another problem that notoriously makes SKS rifles slam fire.

Thoroughly clean out your bolt.

thanks for the info. appreciate it!!

Could be the firing pin issue, but there's another possibility.

The sear on many chinese SKS's is milled at a slightly negative angle in relation to the hammer, so if the sear/magazine release spring (it's the same spring that does both jobs) gets weak over time, the sear can allow the hammer to "slide" off of it, which causes "doubling", or in some cases full mag dumps. The way to fix the problem is to take the sear out and mill it slightly so that it's engagement surface will be at a neutral to slightly positive angle in relation to the hammer. That way, the hammer won't want to slide off under recoil if the sear spring gets a little weak. - Alternatively, I suppose you could try making a heavier sear spring for it, but that would just leave the potential of having the same thing happen again later. YMMV.

Quoted: ...could be garbage around the firing pin inside the bolt that restricts the movement of the firing pin inside the bolt.

+1

This is not uncommon.  I had a Norinco AK that did that once.  Pulled the bolt and cleaned the firing pin hole and found lots of little slivers.  I clean the bolt / firing pin whenever I shoot the gun and haven't had a problem since. (Full-auto is fun, but I'd rather do it when I want to do it, and not the gun feels like surprising me.)

A friend in high school had a Chinese SkS that fired bursts while I was shooting it a couple of times.  

Never happened to my buddy, despite how much he tried to replicate it.