RC,
Fish finders are based upon SONAR technology. In the case of commercial fish finders, they beam an ultrasonic frequency into the water in a cone shape to the bottom. There can be special software that helps a person determine what is a fish, but what it is really picking up is the air bladder in the fish that helps the fish hold whatever depth that it wants. This air bladder is what allows the fish to not sink to the bottom always.
Ultrasound units that people use to diagnose medical conditions work using the same technology, which is the difference in tissue density. Fluid filled organs allow the signal to pass through easily, and show up as black on the screen, while tissue of other densities shows up as a gray color or white, if it reflects a lot of the signal back.
There is a special type of fish finder type device that is based from a normal fish finder, but has special software in it. It is known as a "body-cator" and is made to help find drowning victims or perhaps homicide victims. It works from the same principal as the fish finder, using the gases that are the product of decomposition, and the fact that a weighted or trapped body will be trying to float, and so will have some degree of separation from the actual bottom of the body of water. I am not sure how well they work, but when I was in EMS we had a local rescue unit that had one with their water rescue unit.
Looking for a gun, club, shovel or anything like that would not work with a fish finder, even though there is a large difference in the density between a hard object, like a gun, and the soft (usually) bottom of the lake. This is due to the signal being diffused by the soft mud/weeds at the bottom of the water. You can see some hard objects, like stumps, rocks as solid objects, but something smaller would be hard to tell. Usually, there is a fairly wide band of noise at the bottom of the finder that signifies the bottom, and is caused by the diffusion of the signal.
CyberPro