Probably shooting some Russian ammo?
They probably fired it, the case expanded too much and got stuck in the chamber. ONLY IF it's fired and there's no hot round in the chamber--->
1. take out the magazine.
2. get a metal rod and stick it from the muzzle like you see in the Civil War muskets and drive it home. This should dislodge the case from the chamber.
Sig chambers are probably one of the tightest and thus achieve maximum accuracy. But cheap ammo will get Sig to jam up in no time.