I will try to explain the difference in durability between hammer-mounted strikers and frame-mounted firing pins:
Hammer-mounted strikers' failure modes tend to be chipping on the very tip of the fp, and that is due to the nature of the fp being "wiggly" to allow for manufacturing tolerances between hammer and frame dimensions. These manufacturing tolerances are inevitable, and to keep costs down, the wiggly feature allows the stiker to impact the frame and be guided into the fp bushing, and through the hole. The hammer-mounted stiker design is original to S&W and most other revolvers with a late 19th century design origin. The K22 and kit guns don't use a hammer striker for the simple reason that the soft .22 rimfire case is subject to flowback into the primer hole. The frame-mounted fp prevents that.
The centerfire frame-mounted fp is a much more recent design, having been introduced on the Charter Arms, Colt MkIII and MkIII, Ruger Security Six, and the S&W designs (excluding K22 and Kit guns) of the last ten years or so. It is used exclusively on newer and high pressure revolver designs, notably Freedom Arms, VIrginian Dragoon, Hammerli, S&W 500, etc. The reason frame-mounted fp designs tend to be more durable is because the fp does not have to endure the frame impacts like the hammer strikers. It is through-hardened to withstand both hammer impact and primer thrust. The frame-mounted fp design is more resistant to higher pressures because it is better able to control primer cup flow i.e. closer tolerances for the moving parts. Once a suitable material and heat treat is found, the frame-mounted fp is amazingly durable. Charter Arms made their fp out of beryllium copper, a spaceage alloy formulated for strength and durability.
