you can measure the soundpulse of the cannon firing and the soundpulse when it drops the floor or it hits the wall.
That too will give you two timestamps and a distance.
If you shoot right up in the air you can even determine the velocity of the ball leaving the canon as the ball makes (almost) a perfect parabola.
Gravity will decrease the velocity until it reaches zero
V = V0
- g x t
V = 0 when it is at the top, => V0
= g x t
g = 9.80665 (depends on where you live), and t is half the time between firing and the moment it hits earth again.
so if t = 14.35 seconds => 0.5 t = 7.17 => V0
= g x t = 9.81 x 7.17 = 70.34 m/s
The height the ball would come is 0.5 x g x t2
= 0.5 x 9.81 x 7.172
= 252.16 meter (so don't test this indoors
The above calculations do not include drag / friction.