Mine are barely over 1/8", so probably 3.5mm. Nothing close to the wide open draws on my Ceppos, but they've never felt constricted when I smoke them.They all smoke very dry too. They are among my favorite pipes, I wish I had bought more before Les retired.
They have an open, easy draw, and are my favorite English pipes, maybe my favorite pipes of all. It's a fight between them and my Rinaldos. I have three Ferndowns and one Barber Wharfdale and I have never had a bad smoke from any of them. If they have a fault, it is that they are almost too large/heavy for just hanging from the mouth.