If you choose to sand the inside of the forearm stock, I found that a broom handle, pvc pipe or dowel the same size or a little larger than the barrel diameter wrapped with 60 grit sandpaper works well, followed by 180 to smooth out the roughness.
The dowel bridges the low areas while hitting the high spots and simulates the barrel.
Check the stickies for other methods of adding spacers, washers, set screws etc.
ALSO,as suggested, shoot it first.