I’ve been in the RCMP crime lab and saw a demonstration of one of those water tanks. It was about 4.5 ft deep and about 8-10 ft long. The technician fired the gun into the water at an angle and retrieved the bullet with tongs, pretty much intact (except I would agree with duffer about hollow-points - I would think they would expand due to the density of the water). The only markings on the bullet would be from the gun barrel. They didn’t have a separate tank for long guns, so I assume that the 8-10 ft length was enough to slow down a bullet fired from any gun.
The Teeming Millions are also discussing this sport: Okay, Vermonters, get your asses in here and explain