The vibrations caused by a car stereo system at full blast is enough to trigger a misfire in an SKS rifle.
This myth originated from Russian gangs, who use an SKS rifle with a freesliding (as opposed to spring-activated) firing pin. Supposedly, any large vibrations, such as from a loud car stereo system, can cause the firing pin to trigger and fire off round after round at random. To test this, the Build Team “pimped their ride” with the most advanced sound system they could get, and placed several of the appropriate rifles in different areas in the car before subjecting them to several decibel and tone levels worth of sound. However, none of the guns went off. Even taking the guns for testing at a sound studio and getting a custom car specifically designed for sound system contests couldn’t produce a result.
Tell your friend to stop smoking the ganja. It is screwing up his memory. The above from the Mythbusters web site. Myth de-bunked.
They were able to set off an SKS from a nearby explosion. If you think you might survive the mortar going off 10 feet away from you and are afraid it may cause your SKS to spontaneously fire, go here: