|First step. Remove bolt as usual for an AR15. Separate the bolt from the carrier|
|Locate firing pin cotter pin. It is silver and in the middle of the bolt lined up with the thing that holds the guide rod in place. The dark colored thing on the right is the ejector. Don't mess with this.|
|Original firing pin spring (fps). Sorry its upside down. All measurements in millimeters.|
|Diameter of original FPS|
|Diameter of new FPS|
Reinstall everything in reverse order. Be sure to use a brass punch for the cotter pin so you don't dent it. Make sure that the recess in the firing pin lines up with the cotter pin. You'll have to slightly push in the firing pin while driving in the cotter pin to get them to line up.
ABSOLUTELY MAKE SURE TO TEST FOR SLAMFIRES.
Do this by using a shell that has been fired or better by removing the bullet and powder. Load the shell into the chamber by hand. Rip the bolt back and let it fly a bunch of times. Make sure you haven't just created an open bolt gun.
Lastly, make sure to test some already fired shells or ones with no powder or primer when going back to the old hammer. You may get too much of a strike or pierce the primer with the lighter spring and the OEM hammer.
This is why the firing pin is now inertia driven: http://www.thefirearmblog.com/blog/2010/07/31/sw-mp-15-22-kaboom/
S&W redesigned the FP system after this started happening.
Let me know how it works for you.