You need a good lockup when closed, thats where you shoot from. My guess is the forearm spacer isnt applying much pressure to the front of the action so its loose when open. You can make a shim to glue in between the spacer and the wood for a permanent fix. To 'feel it out' drop the forearm and put a strip of duct tape across the contact area, one strip at a time until you get a firm fit, then you have an idea how much shim you need.