Ted,
I just checked the barrel flats on my Merkels this morning. The 12 ga. is stamped 18.6 - .732, 20 ga. 16.0 - .630, and the 28 ga. 14.0 - .551.
Again, slight over bore on the 12 & 20, with virtually none on the 28 ga. The mysteries of shotgun barrels!
Hope this helps,
Karl