The Master Gun Lock
I'm sure just about everyone has seen these fine trigger locks that Master puts out. They come in two pieces and are shaped like a large steel and rubber kidney bean. One side has a post that fits into a hole in the other. The inside of the lock is a tough rubber that won't damage any finish. There are molded rubber cleats on the inside as well as plastic pins that retract into the lock body if they are compressed from the top. This allows the lock to fit a wide variety of firearms.
Once in place, the lock prevents access to the trigger by enclosing the trigger guard. The cleats and pins on the inside engage the trigger guard and keep the shaft, which passes through the guard, from moving. This keeps the lock from activating the trigger.
While I have never been able to move the lock once it is applied to the firearm I wouldn't trust it enough to keep on a loaded gun. The lock fits almost all weapons snugly except for modern firearms with pistol grip to trigger to magazine well configurations. The lock will still fit through, but the cleats/pins may not lock into place.
This device does nothing to keep someone from loading most guns while it is on. It just keeps you from operating the trigger.
While I find them to be very tough, quality made locks I would prefer something that would not allow one to load the weapon.