400 series stainless steel is very magnetic, while 300 series is not. 304 and 316 are used in corrosion resistant applications, like food and medical. 400 series is not nearly as corrosion resistant, but they are easier to machine, so 416 is typically used in firearms. A stainless steel firearm is very strongly attracted to a magnet. I haven't measured the force, but it seems about the same as for a blued gun. As an aside, some slides are black but underneath are stainless steel (SIG).I wonder how this device would work with stainless guns? Depending on the composition of the stainless, it can have very little magnetic attraction. Some stainless will grab on to a magnet, other stainless will not. Don't know which composition of stainless is used in guns, or if that varies.