I do use a core box bit, but it is only 1/2, so I mark the front of the forend with a line marking the desired dimensions of the channel, less about 1/16th. Then, I cut several passes, adjusting my guide rail and cutting depth to follow the dimensions of the channel. Kind of a PITA, but no more often than I make one, it works pretty well. I clean it up with a brownells barrel channel scraper, and it comes out beautiful. I have the bottom flat, slightly angled shallower toward the front, and a little of the weight removed with the cove bit bit along the sides.
Looks OK, and takes quite a bit of weight off.
Start with a totally square piece, cut the frame end contour first. Copy a regular forend, works good. A scroll saw works great if you aren't going too wide, I used a coping saw and files, very carefully. Cut the barrel channel next.
My next purchase and shop project: Buy a 1/2 inch router, a bigger Core Box bit, and build a better jig. Then it'll be a piece of cake.
Hope this helps,
Dan