Ed,
Having no experience with this type of problem in a longrifle build, I am a little reluctant to offer advice. However, I have dealt with similar challenges as a cabinetmaker and general woodworker.
I would not seal the barrel channel at this point. Either the inletting and shaping have released stresses in the wood or the wood is continuing to dry - probably the former. If corrective action is required, I would start by making a wood filler strip that fits the barrel channel as it should be, and is between 1/32" and 1/16" smaller than the barrel dimensions. The filler strip only has to fill the area between the vertical flats and it is slightly under-sized to compensate for spring-back. Thoroughly steam the forend without the filler strip in place, insert the filler strip and tightly bind the forearm with surgical tubing, large elastic bands or strips of innertube. Being a cheap SOB, I use 1/2" strips of bicycle innertube, which I have found to work as well as anything. Set aside until thoroughly dry. The barrel could be used instead of the filler strip, but I would expect a bit of a gap due to spring-back.
It would be wise to brace the forestock, since it might have a tendency to warp either vertically or horizontally, or twist. Effective bracing could be as simple as a straight piece of 2x4 with a piece of 1" stock scabbed onto one side, creating an "L" shaped channel for the forestock to fit into. clamp the previously-wrapped forestock into the channel to dry. It is always advisable to do an end-to-end dry run in order to avoid those "oh s**t" moments.
This is not a simple process and not without it's risks. If there are only slight gaps, and if you can live with it, you might consider glass bedding with tinted Acraglass or something similar - even tinted bee's wax.
