A big O-ring fell out of my gun when I took of the outer barrel. I have no idea where it is supposed to be installed.
This o-ring goes in between the receiver and Outer barrel. It is there for mass production and for ease of the assembly process as it helps to center the Outer barrel in the receiver before the thread grips. After a couple of assemblies and disassemblies by the customer, it will fall out and might even be broken.
Throw it into the trash. This part has no function once the gun is assembled. We will not put this ring into the SSG-10 starting from batch#3 as it seems to confuse customers too much.