CompNChoke makes and sells tubes as open as -.005". That's negative five thousandths, meaning that it is five thousandths more open than true cylinder. They're thin, and can be damaged by a fall or a hard lick, but if someone wants one they're available.
Beretta tubes marked 'skeet' have been like that at least since 1997 when I bought my first Beretta with 'Mobil Chokes'. They measure about 5 thou over bore. Those tubes are threaded on the muzzle end too for whatever reason, and they had problems early on with steel shot expanding the choked area into the threads making the tubes near impossible to remove. I broke a Brownell's stuck choke tool removing a M and a IM from an early 686 after the owner had fired a few hundred rounds of steel. The thread damage was limited to the last couple threads so the barrels were still serviceable. Beretta changed to a tougher stainless for their tubes and the steel approved ones are now marked 'SP'.
Brownell's refunded the tool. They made no comment, but I would have liked to have heard what they said when they got the thing back.
