Victor3, We haven't even see one picture of your rifled cannon yet and now you are going on to a round shot gun? Well, that's OK; everyone needs more than one cannon, including you. Although we can't tell you specifically what that muzzle velocity would be, we can give you some good news about the use of a chronograph to measure ball or bolt velocities.
You should have good consistent results if you:
1) Cover the light entry ports above the light sensors with clear cellophane tape. This will keep any particulate contamination from the BP combustion process from partially occluding the sensors. We have found, by actual tests, that this is important.
2) Place the sensors far enough away from the muzzle to avoid any blast effect. We found through firing tests that 15 feet works for our application which is 518 grs. BP pushing a 9 oz. steel bolt at 1,300 fps. Plenty of muzzle blast, but fewer escaping powder grains due to the heavier projectile mass with the result of higher pressure producing a more complete burn.
The photo below shows chronograph position during the firing of a rifling-test barrel in our landscape log fixture. The position of all equipment is apparent. The only other tip we can offer is the projectile coloring procedure which we use to prevent glare from sunlight as the projectile passes over the screens. On our 12L14, cylindrical, steel bolts which we shoot "in the white", we use a black marker to color three stripes from top to bottom aligned with the bolt's long axis. On a round shot, we would color the whole shot.
Good luck,
Tracy and Mike
