Back in the early days of the M686 S&W received some complaints of primer flow back into the bushing which seized the firing pin and locked up the gun. As it was explained to me back then, S&W realized that there were some softer primers available on the market which could potentially allow this condition to occur.
The designed a revised bushing & firing pin and then a special tool kit, accompanied by written instructions, for LE revolver armorers to use to remove the old parts and replacement them with the new ones. The barrel did not have to be removed. A punch/stamp was also included to stamp the inside of the yoke with a "M" to designate that the revolver had received the modification.
I have the instructions stashed away somewhere among my collection of manuals, but I don't have the tools.
Personally, I wouldn't attempt it without the tools ... as well as digging out the instructions.
This reminds me. I actually know a retired armorer who still has this special field tool kit and I've sometimes thought about asking him what he would take for it, just to add it to my collection of S&W tools.
