I would say 15 cm is certainly possible with forend pressure. I built a frakenmauser a couple of years ago for hunting with an RC barreled action and a mystery beech stock. I had to fashion a handguard because the stock was 5/8" too short. When I assembled it for testing didn't notice the handguard was touching the barrel on the left causing it to shoot fat right. I drifted the front sight far to the right before I realized the problem.
Good luck