The .50 Cal Ammo Can is about 7" tall, 6" wide and 12" long.
The 20MM Rocket Box is much larger at 17 1/8in x 7 1/2in x 14 1/4in.
There are also a bunch of other sizes of both ammo cans and rocket boxes out there. As mentioned earlier 20 MM Rocket boxes are standard for Groovers. So TMTTR was it a 20 MM?
As to the shooting - if you wanted to send a message - why not walk to the waters edge and shoot into the river where you could know where your bullets were going. It eliminates the risk of hitting someone one or thing unintentionally and really the sound of the gunfire is the primary deterrent. If the people could actually hear the bullets passing over their camp the shots were far to close for any adequate margin of safety. IMHO.
P.S. Have you found my drag bag yet? 'Cause I might have to start shooting up your camps soon