The SKS has an inertial firing pin - that is , it does not have a spring to keep it away from the primer until the hammer strikes the firing pin.It is basically a turned piece of steel with flat spots milled out , IIRC there are two flat cuts on the top and one on the bottom , at least on the Chinese version. It is possible to place it in the bolt upside down and hammer the retaining pin in , leaving the firing pin tip protuding.
BANG , BANG..........BAD ! Uncontrolled firing.
Dried old gummed up cosmoline can also allow the FP to protrude , as well as soft (non military) primers , and a simple burr can cause a semi - any semi - to slam fire.
This is why you should always keep a tight grip on the weapon when chambering a round and test fire ANY semi auto new to you , or newly repaired ( really after each cleaning a function test should be conducted ) .
New to you semi test :
EMPTY WEAPON cycle action / pull trigger / hold trigger / pull bolt back / with the bolt all the way back - is the FP protruding ?
If no , close bolt / release trigger / was the hammer caught by the disconnector ?
If yes weapon is good to live fire.
Load ONE round IN THE MAGAZINE chamber round follow above procedure after firing round.
Everything worked right ? - load two rounds in magazine going through above procedure for each round.
The weapon is functioning correctly / you can load a full magazine.
If ANY of the steps show a problem UNLOAD THE WEAPON AND TAKE IT TO A GUNSMITH.
I had a BRAND NEW Century MISR that had a FUBAR'd FCG that had I simply 'loaded and fired' would have gone FA had I not discovered the problem prior to shooting it. I replaced the faulty FCG and after function testing it passed and I have had no problems with it since.
Guns CAN be DANGEROUS ! Be careful.