SAP Pistol Firing Pin Misfire

Discussion in 'Gunsmithing Forum' started by Richard-Kent, Sep 4, 2012.

  1. Richard-Kent

    Richard-Kent New Member

    I have an Australian Arms Model SAP pistol in .223 caliber. This is a semi-auto with an action similar (maybe identical) to the Australian Arms assault rifle.
    The gun misfired on two rounds out of a total of about 20. The primer was indented but the depth of the indentation was about medium to small compared to other guns I have.
    The ammo was a cheap foreign brand that is at least 5 years old. I did not try to fire the rounds the second time; although, I wish I had.
    The gun was reasonably clean. It had fired no more than 100 rounds since it was new. The bolt is easy to remove and disassemble without any tools required. The hammer spring was never left in the cocked position. I have dry fired it a few times to drop the hammer before storing the gun.
    My basic question is: What is the preferred amount of firing pin protrusion above the face of the bolt. I can easily measure that with a dial indicator.
    The firing pin has a flange at the rear that bottoms on a tubular part of the bolt. This tube section could be easily filed to allow greater firing pin protrusion above the bolt face. I hesitate filing it because replacement parts may be difficult or impossible to get, and I don't have a method to keep the surface perfectly square.
    Any suggestions would be greatly appreciated.
  2. mountainman13

    mountainman13 New Member

    Welcome to FTF. I would suggest trying different ammo.

  3. danf_fl

    danf_fl Retired Supporter

    The mountainman13 is providing words of wisdom.

    Don't fix the firearm until you are positive that the ammo is not the problem.

    Primers of some ammo maybe "harder" than others. Get good reputable ammo first.
  4. c3shooter

    c3shooter Administrator Staff Member Admin Moderator Lifetime Supporter

    Firing pin protrusion will be the same on all rounds. If you have 2 out of x with shallow indentations, the cause is likely the hardness of those primers, and not the firing pin.

    AR 15 armorer manuals use a guage with a GO dimension of .036" and a NO-GO dimension of .028".

    With the firing pin held firmly forward in the bolt, the end of the firing pin protrusion gauge (or gage, if you prefer) marked "MAX" (GO - 0.36") should pass over the end of the pin without touching it, and the end marked "MIN" (NO-GO - .028") should hit the end of the firing pin and not pass over it.
    Last edited: Sep 4, 2012
  5. Richard-Kent

    Richard-Kent New Member

    Thanks for the information. Yes, the problem is the ammo or rather the primer in the ammo. The gun is right on spec.
    c3shooter, I checked the firing pin protrusion with the "hole depth rod" feature on my dial calipers. The dial marks are 0.001". It worked nicely and was quick. The firing pin protrusion is between 0.035" and 0.036". This is at the top of the range listed as 0.032" to 0.037" by one reference. The indentation in the two primers by a quick glance appeared to be the full amount of the protrusion, but I no longer have the two defective rounds. I guess the defect in the primer could be anything. They were never exposed to moisture, that I know of.
    Yes, I will use different ammo for serious shooting, and the defective ammo can be disposed of by punching holes in targets and cans.
    Replies are very much appreciated.
    I consider this case closed.