As was said, the barrel at this stage of the build should slip fairly easily into the barrel channel.
Do
NOT use a chisel to remove wood from the 2 sides of the barrel channel.
It will create large gouges where it has removed too much wood.
Get a small piece of a 1 X 2 board or a piece of wood that will easily fit down into the barrel channel. (A 1 X 2 board is actually 3/4" X 1 1/2").
Get a soft lead pencil.
Keeping the sides of the barrel parallel with the stocks barrel channel, lightly start the barrel into the groove.
Notice any locations where the wood seems tight and look for any locations that look loose.
Use the pencil to mark the tight areas.
Use some 80 or 100 grit sandpaper on the block and carefully enlarge the width of the channel in the areas that were tight.
Keep the block square with the world so it does not 'bell mouth' the groove.
Work slowly and patiently, stopping to check the looseness of the barrel and remarking the tight areas.
When you have the clearance right, you won't be able to actually see a gap between the wood and the barrel but the barrel will (if it is lined up properly) fit easily into the channel.
Once this fit is finished, you can start working on the bottom flats of the barrel channel by using the inletting black to identify the high points.
Follow this link to see the gun in the bottom of my posts
http://www.muzzleloadingforum.com/fusionbb/showtopic.php?tid/84290/post/84290/fromsearch/1/