If you have the manual for the GPR is has a section on fixing that problem. Bending the wedge pins is one way of doing it but I don't think that way is in the manual. The manual shows using a piece of round stock (like a metal bar) and slightly bending the wedge slot of the barrel for a tighter fit. I used that method on one of my barrels so I wouldn't have problems with my other barrel and a bent wedge pin.

Good luck with it.