As a lot of the folks said, the barrel channel will close up and the gaps will go away.
That said, I do draw file the barrel before inletting it; top five flats only. I draw file just to get the machine marks out. So, if the barrel was well sanded, just a couple stokes may do. Also, draw filing properly (well chalked and frequently cleaned file/surface) will give you a surface as good or better than 320 grit paper.
As Tom said, I also make sure my breech plug is good before inletting. I pull it to install the liner, so that is the time to make sure everything is right. I prep all my parts before stocking and that includes the barrel. I make sure just about everything is ready to inlet before I start stocking. If you do that, then you can get everything together in a couple weeks or less.
I learned all this stuff after many mistakes.