If I'm not mistaken, its in the Stock and Action compartment, not the barrel compartment. The reason for saying this is that the cover buckles are on top, with the cover hinges at the bottom and it looks like the barrels go in the top compartment?
I therefore guess its there to allow adjustment for stock length, so that the stock and action doesn't slide backwards and forwards in transit. That's a guess though!

Last edited by JohnfromUK; 11/19/13 12:17 PM.