The scoring isn't as bad as you think. Without looking at it I'll bet it's the same minor marking as most have. Your piston head could be slightly undersized. If you search here a bit you'll find the correct outside measurement for them. If they fall below this you can get the sympton you describe except I should think it would be far more often than you have it. Is your gas cylinder lock screw tight? Is there proper lube? These are simple things that won't cost to fix. A new gas cylinder isn't expensive either. You could get the piston head redone too, cheaper I think than buying a new one. If it's a used piston, you may well run into the same confusing problem. Don't worry, we'll get it sorted out yet.