I have the BB column molds- both kinds (the PVC w/liner, and the silicone).
The PVC has an internal dimension of 3", and the liner fits with such perfect precision all the way around that there's only a very small trace of a seam line down one side the finished soap. For what it's worth, the tight-fitting liner doesn't make things very easy when it comes to unmolding the soap. At least not for this little slip of a girl, at any rate. I always have to use a 'pusher', which for me (like Cindy) is a glass jar. When I first bought it, I thought the 3" might end up being too much, but my soap actually shrunk down enough during cure to just fit snugly in my hubby's mug, which is an old coffee cup.
The ID of their silicone column mold is 2.75". It's much easier to unmold than the PVC, and it's shorter, which works out better with my smaller-sized batches. It also snaps together wonderfully tight so that I've not had any leakage except for that one time when silly me banged the mold on the counter much too hard in order to eliminate air bubbles and the thing came apart (yep- you might want to watch out for that, lol ) Sure, the round log comes out with 3 seam lines (2 sides and the bottom), but this is not a big issue with me.
Lately, though, I have not been using either one for my shaving soaps because I've been making those very pliable KOH/NaOH croaps which I form into balls to cure and store, and then I squish them up to fit inside hubby's mug nice and snug whenever he needs a re-fill.
IrishLass