My thoughts, my understanding this was a pre-carve? I also assume you put the barrel in after squaring up the breech, but before inletting the breech plug or installing the barrel tenons? How did it fit then? I'd remove the inletting black with a sandpaper backed by a small block of wood as mentioned above. Then I'd check the barrel tenon's clearance especially the one nearest the muzzle. I can't tell from your pictures if that's inletting black around your tenons or pencil. Make sure you blacken the tenons. If it was me, and the breech was tight and no gaps along the side walls, I'd do as Mike says, move on. But its your gun so take your time with it and learn, even if that lesson is not to sweat the barrel fit so much.
Note: don't be surprised that the barrel isn't 100% even if it is a pre-carve and especially if they routed the ramrod hole rather than drill. To me, and this is just my opinion, routing a ramrod hole is a short cut.
My son recently started his first gun. Like most first timers, everything is new and he tries to over-think things. Being particular and learning the proper workmanlike techniques is important. Understanding the way things flow is important. Understanding what is important is important.
But workmanlike doesn't mean perfect - it means "showing efficient competence". I want him to go slow, learn and enjoy. What I want him to learn about the barrel channel is, get the breech tight, get the sides even and tight, no gaps, don't spend a lot of time making the channel match the octagon barrel except at the breech and as close as you can without losing sleep at the muzzle.