Hey guys. Not a Gamo question, but I figured someone may know.....

I just took apart a Powerline 1000 springer,...first time disassembling any springer, so I don't know exactly what things should look like. The front of the piston has a hollow cavity, about 1/4" deep and maybe 3/16" in diameter or slightly less. What exactly is the purpose of that? It's obviously machined that way for some reason, maybe to detune the gun slightly, I don't know. I thought pistons were generally  flat or the seal slightly concaved (for the type that cover the whole front of the piston). Also, I'm wondering if filling the hole somewhat would make the gun shoot less harsh, by increasing compression a little and giving a little more cushion at the end of the piston stroke. Any input is appreciated. Thanks!