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M16 Auto Sear Question

Discussion in 'NFA Firearms and Accessories' started by TMiller556, Aug 22, 2011.

  1. TMiller556

    TMiller556 Well-Known Member

    I understand that in an M16, when the selector is switched to automatic, the disconnector is held down by the cam on the selector, but what causes the auto sear to release the hammer? By the way, why is the auto sear necessary? I never understood this. Thanks in advance.
  2. Art Eatman

    Art Eatman Administrator Staff Member

    All I know about it is that the auto sear converts the AR from semi to full. I don't know if semi-auto can still function.
  3. Telekinesis

    Telekinesis Well-Known Member

    Are you talking the real auto sear as in the one that keeps an automatic rifle from firing out of battery? or the lightening link type of transferable auto sear?

    I'm not positive about the auto sear on the M16, but other auto sear designs I've looked at are engaged (will hold the hammer back) while the bolt is out of battery (as it is while it is cycling) and then disengage (allow the hammer to fall) once the bolt is back in battery. It is very much like a disconnecter, except that it ONLY functions while the bolt is out of battery. This lets you use higher pressure ammunition without worrying about lighting off a round before the chamber is fully locked.

    Early SMGs (the ones that operate more or less with an open bolt "slam fire" type of process) didn't use a auto sear because they could survive the relatively low pressure of a round if it was fired out of battery. They also don't really have hammers, but that's another point entirely.

    Another use is to slow the rate of automatic fire.
  4. TexasRifleman

    TexasRifleman Moderator Emeritus

    If you have 20 minutes, here's an old DoD video that explains the whole thing. Very basically, the auto sear allows for the hammer to be reset to the main sear surface without releasing the trigger. On semi auto the trigger has to be released to get this reset (to prevent slam fires). On full auto the trigger can be held down and this secondary sear surface releases the hammer to the primary sear surface without the trigger having to be released.

    When you switch the selector to semi the auto sear is removed from the equation, meaning that now you have to release the trigger to get the hammer reset onto the primary (firing) surface.


    As to what releases the primary sear instead of the trigger, this picture will help. The bottom is a full auto bolt carrier, the other 2 are semi only. Notice any difference? :)

  5. ogree

    ogree Well-Known Member

    When the weapon is set to "full", the bolt carrier trips the auto sear releasing the hammer just as the bolt locks up.
    The auto sear it what retains the hammer in the cocked position while in auto mode and not the primary sear engagement, the secondary hook on the M-16 hammer is caught by the auto sear during the period while the weapon is cycling when the trigger is in the rear position.
    As soon as the trigger is released the primary sear engagement takes over again.
    Last edited: Aug 22, 2011
  6. TMiller556

    TMiller556 Well-Known Member

    oh okay i get it now.. so you mean the purpose of the auto sear is to make sure that the bolt is in battery at the time of firing = so the breach is closed at time of firing = slowing down cyclic rate?
  7. TexasRifleman

    TexasRifleman Moderator Emeritus

    Well, sort of.

    The secondary sear catches the hammer so that it doesn't follow the bolt down. The auto sear just releases the hammer to fire the rifle instead of you pulling the trigger, the bolt carrier pulls the trigger for you in a manner of speaking. Holding the trigger down allows the auto sear to work so you don't have to. If you let go of the trigger the auto sear is mechanically taken out of the equation and the secondary sear drops the hammer back to the primary sear surface again. At this point you have to pull the trigger at least once to get the auto sear working again.

    Cyclic rate is more a function of the buffer, bolt carrier mass, and spring weight.

    If the rifle could fire before the bolt is locked into place you might get a faster cyclic rate but you'd eventually blow up the gun.
  8. TMiller556

    TMiller556 Well-Known Member

    So if the selector is on automatic and you fire single shots, the auto sear doesn't catch the hammer? I thought that once the selector is on automatic, the auto sear automatically pops out. What engages the auto sear?
  9. TexasRifleman

    TexasRifleman Moderator Emeritus

    Holding the trigger down allows the auto sear to function. If you let go of the trigger the firing sequence stops, even if the selector is in "auto".

    With the selector in "semi" the auto sear can't engage at all. You can fire semi with the selector in "auto" all day long if you are fast enough on the trigger release.

    All the auto sear really does is drop the hammer for you at a safe time if you are holding the trigger down. The horribly long and boring video I posted does a really good job of explaining it, if you can stay awake during the 1960's narrators monotone :)
  10. ogree

    ogree Well-Known Member

    With high pressure cartridges you want the bolt fully locked up before the round is set off, otherwise the case would rupture if it was not fully in battery (Chambered) resulting in an unfortunate problem. (BOOM)
    The manner in which this accomplished in the M-16 has nothing to do with cyclic rate.
    Like TexasRifleman said "Cyclic rate is more a function of the buffer, bolt carrier mass, and spring weight".
    Last edited: Aug 22, 2011
  11. ogree

    ogree Well-Known Member

    If you are pulling off single shots while the weapon is set on full that means you have a lot of practice with that rifle and have a very quick finger.
    It is not unheard of but in that case the auto sear would not engage and the primary sear would.
  12. TMiller556

    TMiller556 Well-Known Member

    Lol actually, I've watched that video a while ago, but I'm going to check it out again since I've learned a lot since the last time I saw it and hopefully it will all make sense. Another thing that confused me is how the bolt cam pin works, but I'll look for that in the video. By the way, what was the secondary sear that you mentioned before? Do you mean the nose of the trigger or is there another sear?
  13. TexasRifleman

    TexasRifleman Moderator Emeritus

    I probably used the wrong term calling it a "secondary sear". What I mean to describe is the method by which the hammer is stopped from following the bolt. There's an extra hook for that on an M16 hammer, allowing for the auto sear to hold the hammer til the bolt carrier whacks it. On the semi auto only the hammer is held by the disconnector, which then drops it onto the primary sear surface when the trigger is released.

    Auto sear:


    It just "fires the hammer" so to speak, and it's why even having a receiver with that extra hole drilled is a felony without a tax stamp, because it's such a simple little thing.

    Then you add the rotating cam mechanism for 3 round burst and it gets even more weird :)

    Here's another good resource that shows this.

  14. TMiller556

    TMiller556 Well-Known Member

    That's what I thought you meant, just wanted to make sure. Anyways thanks for the help, you've cleared a lot of things up for me. And thanks for sending the video to me, I understood it now better than I did last time.
  15. MrCleanOK

    MrCleanOK Well-Known Member

    The cam pin is actually pretty simple. When the rifle cycles, combustion gas from the bore travels through the gas tube, and into the expansion chamber of the bolt carrier group. The bolt itself is the piston inside this chamber, and the gas rings make the seal. The expanding gas pushes the bolt carrier to the rear, while the bolt remains stationary (the gas is actually pushing it forward). As the bolt carrier moves to the rear, the cam pin rotates the bolt so that the locking lugs can clear the receiver. As the excess gas vents from the carrier, the momentum of the carrier takes the bolt and the empty casing to the rear with it.
  16. TexasRifleman

    TexasRifleman Moderator Emeritus

    Yes, and thanks for the writeup on that. That one confuses people too for some reason. When I said the cam was weird I meant the cam for 3 round bursts. There is an additional rotating cam added to the FCG to do the 3 round burst. It basically interrupts the auto sear after the third round, requiring the trigger be reset before starting the sequence again. If the sequence is stopped before all 3 shots are fired, say by letting off the trigger after one round, then the sequence must be completed before another 3 round burst can start. So in the case of interrupting after one round, another trigger pull only gets 2 rounds fired. That rotates the cam back to the beginning so you can start the 3 round sequence again.

    An excellent description of the 3 round burst sequence here:


    This is quite a bit different from the HK 3 round system which resets after letting go of the trigger, so you always get 3 round capability even if you interrupt the sequence.

    The M16 design is really pretty amazing. It's very simple to manufacture compared to HKs etc. and quite hard to break.
  17. TMiller556

    TMiller556 Well-Known Member

    wow once again, thank you. i was looking all over for something like that yesterday and all I was able to find were pictures of the cam which didn't help at all. i never even noticed how complex an m16 is, especially the a2 and a4.
  18. MrCleanOK

    MrCleanOK Well-Known Member

    TMiller asked specifically about the "bolt cam pin". The cam pin and the burst cam are two unrelated things. M16/M4's are fun little machines :)
  19. Jim K

    Jim K Well-Known Member

    I tried to wade through all that and I am just confused.

    Part of the key to understanding is that the auto sear works all the time. Everytime the M16 bolt carrier goes into battery, it trips the auto sear, regardless of selector setting. But if the selector is set to semi, the hammer is being held back by the primary sear at the bottom. When the trigger is reset and pulled again, the carrier is forward and the auto sear is disengaged, so the hammer falls.

    Setting the selector to FA disengages the primary sear as long as the trigger is held back. That lets the hammer free to follow the bolt down. But it is stopped by the auto sear engaging the notch on the top rear of the hammer, so the hammer won't be released until the carrier goes into battery and trips the auto sear.

  20. TMiller556

    TMiller556 Well-Known Member

    actually I typed it wrong because I got mixed up, but I did mean the cam in the fire control group but thank you for helping me anyway :)
    Last edited: Aug 23, 2011

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