you really need to file the slot for the bolt in the barrel liner first, to avoid over enlarging the block hole (if you do it installed) and introducing any potential movement. file and recheck frequently. just do the maths and make sure you don;t get too close to the bore ! once it's all happy, glue the liner in with the bolt in place, barrel in the jaws, to ensure no alignment issues. 24 hours later remove the barrel and dress as needed. I normally leave the breach end around 0.5 - 1mm proud - it's easy to trim off, and provide a slight (not too much at all) lead in. also if you need to turn the breech OD down for 3mm or so to hold the breach seal o-ring, now's the time