Literally boring with a bar made for the purpose. It takes a while to widen the chamber from 1/2″ to 1,1/2″ but the bar leaves a nice smooth surface.
It helps sometimes to make things in batches, especially the bellows. For the pipes themselves I’ve discovered that it’s too tedious to make multiple sets at a time but it’s enjoyable to make the bags and bellows that way. Here’s a newly finished mainstock with one of four bellows finished last week.