See
forend fit in the FAQs sticky.
I use a 1" sanding drum in my mill to fit synthetic forends, the diameter which is actually 1.1" is perfect to not change the diameter of the built-in spacer of synthetic forends, sand a little, then check fit, repeat until it fits good, not too tight, not too loose, be sure to keep the sanded surface square to the frame, otherwise it could be too tight on one side, loose on the other. Be aware that changing a forend to fit on barrel and frame isn't a good idea if there are multiple barrels to be used with the forend as it won't fit the original barrel and frame, this is particularly important for rifle barrels, shotgun not so much. This is why many here that have switch barrel H&Rs have a dedicated forend for each barrel. A little bee's wax or paraffin on the frame to help lubricate the mating parts helps a lot.
Tim