Tuesday, November 15, 2011

S&W M&P 15-22 Firing Pin Spring Swap for the Timney 3lb Trigger

First step. Remove bolt as usual for an AR15. Separate the bolt from the carrier
Separate the bolt from carrier by pressing reward on the bolt. You can see the little slot on the carrier lines up with a slot on the guide rod. While pushing reward on the bolt, slide the rod up using the notch to free it. Retrieve spring that will fly across the room if you don't catch it.

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.

This is the other side of the firing pin retaining cotter pin. Use an appropriately sized punch or broken allen wrench to tap it out. I suggest using brass, since these roll pins are easily damaged if you use an undersized punch. Tap it out to the other side.

After you have punched the cotter pin out the firing pin and spring should come free. Remove it and its spring. The guide rod guidepost will also fall out. It has holes in it for the firing pin and cotter pin. These will need to be realigned to reinstall the parts.

Original firing pin spring (fps). Sorry its upside down. All measurements in millimeters.

Diameter of original FPS

Diameter of new FPS

Length of new FPS. Notice the number of coils per cm. Much less, therefore less pressure necessary for the spring to be compressed. Picked these up from the local ACO hardware. Made by Hillman. DO NOT bother going to Lowes or Home Depot, they don't have them. I bought 6 in case I needed to experiment with them.

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.


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.


  1. I have a newer version of the M&P 15-22 with the blue bolt spring. Should I still invest the time into replacing the firing pin spring if it is the newer version?

  2. Only if you are getting light strikes with a aftermarket reduced trigger pull. These instructions are meant for the reduced hammer setup. If you are getting good ignition then don't bother, its not worth the time.

    If you install a weaker spring in the standard trigger set up you can risk too hard of strikes or piercing the primer.

    If you get light strikes with the standard set up, you need to talk to S&W about getting it fixed.

  3. Thanks for this post. I have a 15-22 with a slide fire that I was getting light strikes on while bumpfiring with the stock. Sure enough a trip to the local Do It Best hardware store and $0.40 later I have a fully functional rifle while using the stock.

  4. Don't waste your time reading any other posts - this fixes the problem. After i dropped in my Timney i was getting 7-8 round bursts and then a light strike would cause me to have to eject the round and start again. After replacing the spring with a less dense coil the strikes on the ejected shells are much deeper and stronger. I ran through full mags with no problem at all. Thanks again for posting this excellent instructional page.

  5. Searched high and low and can't seem to confirm if I have the right spring. Please let me know if this is it.

    1. That was the right spring for me. I found it at Ace Hardware for 69 cents a piece and bought two just in case I lost one...which I did, went flying at mach two somewhere...safety glasses folks. I had to clip the coiled ends off to make it fit and not bind and made sure the ends were pointed just end so the ends didn't poke outward just a hair. Be careful putting the roll pin back in folks.

  6. i have a m&p ar 15-22a and I need a firing pin for it where can I find one at?

  7. Simply awesome post.
    Had alot of issues with the 3lb timney on my 15-22.
    Got the spring ftom greggs distributers
    Simple fix. However you fo need to cut the top coil off for fit.
    Works 100% with bulk ammo now!
    Thank you very very very much.
    Message me if your in Calgary. Ill buy you a beer!!!!!!

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