This is my muzzle true-ing tool. There are many like it, but this one is mine.
My true-ing tool is my best friend. It is my only hope to true up my muzzle when I cut it. I must master it as I must master my cut muzzles.
My true-ing, without me, is useless. Without my true-ing tool, I am useless. I must file my shortened muzzle true. I must shoot straighter with a true muzzle
When building a component set ( which is about the limit of my abilities) I often find that getting the right caliber, weight, handling and carry feel into the stock I want depend on cutting down a barrel.
Once I cut the muzzle in a customization project, I stuff the six inches or so of a appropriately sized dowel right out of the hardware store bin into the cut off piece. That way I can place it in the muzzle and true up the face based on where it meets the cutoff and the light comes through or the color transfer shows.
I have found tools like this for .50, .54, .58 and .62 calibers.