And, before the 'Haven't Done It' bunch blows the thread up,
Since you are lapping a VERY short area on one side of the barrel, right behind the muzzle,
Do the lapping the same way gunsmiths did with screw in front sight, where the drill/threading went into the bore for the last 200 years...
Simply use a hard wood dowel rod undersized for the bore, sand a spot down on ONE SIDE, use lapping compound on that sanded spot, use the far side of the crown for leverage.
Wood won't damage the crown in even the slightest.
On larger bores, you can angle sand, then attach leather to the end of the dowel rod in the width you want to deburr or lap.
The length of the lap depends entirely on the length of your stroke, the width of the lap depends on the width of your lapping tool and how well you control the lapping tool.
I go slow, use fine lapping compound so even if I do slip, it makes zero difference.
The lapped area will tell you where the high & low spots are with just hardwood & lapping compound.
Work the high spots! Most try to work the entire area, and you will spend the rest of your life trying to chase out low spots since lapping will show you every single imperfection in detail...
Work YOUR edges, and nothing else or you will loose your mind!
Keep in mind, you can cut your dowel rod to follow INSIDE the rifling grooves, not that you have to, the grooves are the least concern...
Ridges (Lands) are not even that big of a concern since it won't take long for bullets to remove any protruding material.
Keep in mind, it's at the edge of a hole, metal is easy to move when it's unsupported on one side...