The Colt Single Action Army had a bad feature in that if the chamber under the hammer was loaded, the gun could accidentally fire.
Dropping the gun on the hammer or something striking the hammer were things which could cause that.
Early Rugers were the same way but Ruger changed the design some years ago so you can safely carry all chambers loaded. They also would update any of their old models to the safe system and return your original parts for collectors value.
Ruger did that after a shooter was killed with one of their early models. I suppose they were sued.
Most modern single actions have some type of device for keeping the hammer and primer from meeting unless you mean to.
thanks