Read the tech info on the home page. It shows the type(s) of gadgets & cutters i used to open up the channel. You want to be able the slide a piece of thin cardboard or folded index card up the barrel & between the channel & barrel.
Leave plenty of space as this may vary when you're loading a bipod or resting it on a rock or something.

Coarse sandpaper then gradually finer does a good job. Just take a little off at a time & check it to make sure you have enough clearance.
You can see all of this in the pics I put up.

Frank in Fla