Here you go.
Ps newer ones.Tommy/longbow etc are 15.75..
Measurements taken from my own guns
Vulcan mk3 ks. 16mm.
Vulcan mk1 SE. 14.25mm
Vulcan mk3. 16mm
Vulcan mk2 deluxe. 16mm
Vulcan mk2. 14.5mm
Victor. 14.5mm
Hawk mk1. 14.5mm
Hawk mk2 15mm
Hawk mk3. 14.5mm
Beeman c1. 14.5mm
Also if your turning your own the cutout for the barrel latch is generally 9.75 but to be sure use start at 10mm then fit and check for barrel/tube alignment.
I've done a few of my own.