So I've had my new Besson Prestige 2052 about a week now, and I still have a borrowed Willson 2900 here, I was curious about bore sizes, so I pulled out my (plastic) digital calipers. Here's what I got.
2nd valve slide
Besson .590
Willson .590
Compensating loop, 3rd valve slide
Besson .630
Willson .630
4th valve slide
Besson .650/.650
Willson .630/.670
I verified by trying the slides on each other. Other than being different lengths, the 2nd valve and compensating valve slides are interchangeable between the two instruments with a good fit. The smaller leg of the willson 4th valve tuning slide fits perfectly into the compensating tuning slide on the back.
Willson quotes their bore as being .590/.630, Besson has historically quoted their bore as being .580/.640, but has recently switched their literature just to say .590.
Anyways, I had always assumed Willsons to be much bigger bore than Bessons based on the quoted specs, but it's really not true. They are exactly the same until the compensating loop comes out of the 3rd valve at which point the besson starts getting bigger first whereas the willson stays cylindrical until the bow of the tuning slide at which point it gets a little bigger than the besson does in the tuning slide. Their average size is the same.
I also had an interesting time measuring my 4-valve besson 2056 baritone against a friend's besson 955 3-valve compensating baritone recently. Besson quotes the 2056 as being .540 bore and the 955 as being .916. That sounded wrong to me since they play so similarly and have very similar sounds, but that's a huge difference. Well, it's true. The 2056 is indeed .540 on both the 2nd valve slide and the 3rd valve compensating loop slide and the 2nd valve slide on the .955 is indeed .516. The main tuning slides are identical, though, and it looks like the branches and bell from the tuning slide on out are identical. My guess is that they found the smaller bore didn't work so well with the extended low range, and found a way to compensate for the bore with the leadpipe tapers to make them sound and feel the same.
So anyways, this is all very nerdy, but my point was that you can't really compare the bore sizes that manufacturers quote in their literature or on their web site, and even if you can directly compare them, it doesn't necessarily mean anything about how the instrument actually plays!