View Full Version : Any open bolt rifles with a fixed firing pin?
HansTheHobbit
10-04-16, 22:53
This is undoubtedly an odd question, but I was just wondering, does anyone know of an open bolt rifle with a fixed firing pin? I was just wondering if it was mechanically possible, or if anyone had ever tried it.
My prediction is that the primer would detonate before the bolt fully locked into place by whatever means it employed to delay blowback, whether that be a gas system or some kind of inertial lockup. It seems like the risk for out of battery ignition would be too high, or???
ETA: I'm talking specifically about calibers that preclude a simple blowback mechanism, like .223 or .308.
I don't think so. The design itself almost mandates a blowback setup and .45 ACP seems to be about the pressure limit to avoid out of battery discharges and a recoil spring that looks like it came off a Chevy.
This is also why most fixed firing pin SMGs have a bolt that pretty much encloses the entire chamber area. And I've seen what happens to .223s and .308s when they run away.
One case was a sliver of brass wedged into the firing pin of a Galil .308 which quickly resulted in an out of battery discharge that blew the top cover off the rifle and blew the contents of the magazine out of the bottom of the magazine body.
The other was some home rigged attempt at full auto on an AR-15 that clearly didn't go as planned. The upper receiver failed at many points from what I'm assuming was an out of battery discharge.
From a legal standpoint, I think any open bolt firearm manufactured after 1981 is considered a machine gun regardless of actual function.
HansTheHobbit
10-05-16, 01:40
It would have to be delayed blowback at least. Like a FAMAS for example.
What I'm thinking is that the primer would ignite slightly before the bolt went into battery, preventing the locking mechanism from locking up the bolt before the pressure sent it rearward. Let's say the firing pin protruded about .032'', then ignition would likely occur while the bolt was maybe .016'' out of battery. But would the bolt have enough momentum to go ahead and close and lock before the pressure overcame its forward movement?
Also, you could loosen headspace to the outer limit, thereby giving the locking mechanism a chance to engage a little bit early since it would have some slop in it.
BTW, this is purely hypothetical. I have no plans of fixing any of my firing pins. I'm just curious if anyone ever tried to do it. With all the wacky experimental guns in the last hundred years I would kind of be surprised if it was never tried at least once.
Hmmm, M-249. Open bolt, pretty sure it's fixed firing pin. And from fuzzy memory, the M-60 and 240B are as well
To be clear it's not a one piece bolt/firing pin like many sub guns. But it is a fixed pin, not actuated by a hammer, etc.
Likewise, though the bolt does lock, it's not like an M-16.
Better explanation: 249, 240, etc are considered fixed firing pin as the firing pin protrudes when the bolt is fully closed. There is no way for a round to be fully chambered (bolt closed) without it firing.
The nuance in these designs is there is a brief period where the pin does not extend through the bolt face, thus delaying ignition a bit. But they still depend some on advanced (before lock up) primer ignition. It's not a separately actuated motion by a spring or hammer, thus the "fixed" description.
Likewise, Many open bolt "fixed firing pin" sub guns are not one piece, though they act the same as the one piece designs. Ie: they are separate parts, but are attached to the bolt and do not move separately
The other big difference is that the MGs mentioned "lock" in some form, and are not simple blowback designs. It's just not a rotary bolt like the M16 / M14
The Lewis gun has the firing pin fixed to the op rod and the op rod rotates to bolt locking it and firing it. The FG42 and M60 bolt copied this almost exactly. The Stoner 63 series also utilizes a fixed firing pin when configured in open bolt mode.
The BAR is not a fixed firing pin, it has a hammer that hits it that is part of the op rod assembly which I assume the M240 has as well since the mechanism is an inverted BAR.
BTW SMGguns repro FG42 utilizes a fixed firing pin even though its a semi but it does not fire from an open bolt.
HansTheHobbit
10-05-16, 10:23
The Lewis gun has the firing pin fixed to the op rod and the op rod rotates to bolt locking it and firing it. The FG42 and M60 bolt copied this almost exactly. The Stoner 63 series also utilizes a fixed firing pin when configured in open bolt mode.
The BAR is not a fixed firing pin, it has a hammer that hits it that is part of the op rod assembly which I assume the M240 has as well since the mechanism is an inverted BAR.
BTW SMGguns repro FG42 utilizes a fixed firing pin even though its a semi but it does not fire from an open bolt.
How is that even possible???
HansTheHobbit
10-05-16, 14:45
The Lewis gun has the firing pin fixed to the op rod and the op rod rotates to bolt locking it and firing it. The FG42 and M60 bolt copied this almost exactly. The Stoner 63 series also utilizes a fixed firing pin when configured in open bolt mode.
The BAR is not a fixed firing pin, it has a hammer that hits it that is part of the op rod assembly which I assume the M240 has as well since the mechanism is an inverted BAR.
BTW SMGguns repro FG42 utilizes a fixed firing pin even though its a semi but it does not fire from an open bolt.
So these are non reciprocating firing pins, as in they are fully extended while the bolt is going into battery?
So these are non reciprocating firing pins, as in they are fully extended while the bolt is going into battery?
The firing pins are fixed in place in relation to the moving parts assembly. The bolt rotates on its own can path as it locks into battery. The firing pin is moving forward with the op-rod group, through the cam path. When the bolt is fully locked in battery, the firing pin protrudes enough to initiate primer ignition. At least that is what happens in the FG42/M60 and M249/PKM, in a nutshell. Essentially the same thing is happening in all open bolt guns that have a locking system, the only difference being the mechanism for locking. All the same blocks get checked along the way though.
So these are non reciprocating firing pins, as in they are fully extended while the bolt is going into battery?
The firing pin does reciprocate as does the bolt obviously. And normally as a group. But cams change the spacing/position of the parts in that group relative to each other, thus allowing the firing pin to protrude just before the cartridge is fully chambered and the bolt "locks".
But it's still the mass and inertia of the bolt assembly that detonates the primer. And in most cases, they (bolt +firing pin) are moving at the same speed relative to each other just prior to contacting the primer.
It's not an exact analogy, but think how an AR bolt moves with the carrier. The cam track influences bolt position relative to the carrier and firing pin.
HansTheHobbit
10-05-16, 18:02
I watched as much detailed material on the FG42 as I could find, and as I understood it, it's essentially striker fired. If I'm visualizing it right, the bolt fully closes before the firing pin can move forward far enough to strike the primer, or?
Just hypothetically, let's say you modified a FAMAS, or even a CETME for that matter, by welding the firing pin in place and making it fire from an open bolt. Would it work, or would you run the serious risk of an out of battery primer ignition? And if the primer did ignite, would the bolt have enough momentum to close before the pressure sent it rearwards?
I watched as much detailed material on the FG42 as I could find, and as I understood it, it's essentially striker fired. If I'm visualizing it right, the bolt fully closes before the firing pin can move forward far enough to strike the primer, or?
Well, it appears that most of the descriptions are not entirely accurate. If this visualization of the 249 is correct, I'd agree with you that the 249 is essentially striker fired in the sense that it's a separate motion than the bolt at the time of ignition.
https://youtu.be/cAH3fvWIq1A
The fact that the firing pin is not controlled by the sear probably means we need a new term to describe it. But it's not what I'd call a fixed pin as the bolt is stopped but the pin keeps moving to ignite the primer.
Don't know about the others, but the 249 is pretty clear anyway.
edit: Also the M-60 is also the same: https://youtu.be/pULrOj8T5r0
It might be more accurate to say the firing pin is fix in position in relation to the colt carrier. It is the bolt that moved away from the firing pin thus the firing pin only protrudes when the bolt is locked and the bolt is farthest back in relation to the bolt carrier.
At the 30 minute mark you can see how the firing pin on the Lewis works and pretty much many other LMGs after it like the M60 and FG42.
https://youtu.be/FlsEmE5pM10
Near the 7 min mark in my video you can see how the stoner 63A's fixed firing pin works with the bolt locking.
https://youtu.be/-Q3eb0zbBP0
HansTheHobbit
10-06-16, 00:20
Okay, I see what you mean now. I'm not exactly sure what that would be called, but definitely a far different animal than a truly fixed firing pin. Regarding my question, it effectively is more or less the same as the firing pins on any other rifle.
So how about it: What if I were to take a FAMAS, weld the firing pin into place, and make it fire from an open bolt? Would disaster ensure, or would the bolt have enough momentum to go ahead and lock up?
Like I said before, it only has to travel maybe .012'' in between the time that the primer is ignited to complete lockup. Would that be enough time for pressure to build to the point where it would cause catastrophic failure? If my calculation is right, then that would be .000008 seconds, or 8 millionths of a second. From the time the primer is ignited, how long before the powder begins to burn?
You are basically asking if the bolt face can have a firing pin fixed onto it like an open bolt subgun. No it would end up blowing up. You would either have to have excessive head space to try to keep it from firing while chambering and thus case head separation and miss fires or you would ahve ignition out of battery ot only partially locked.
Just hypothetically, let's say you modified a FAMAS, or even a CETME for that matter, by welding the firing pin in place and making it fire from an open bolt. Would it work, or would you run the serious risk of an out of battery primer ignition? And if the primer did ignite, would the bolt have enough momentum to close before the pressure sent it rearwards?
In my experience no. It would run but it wouldn't be timed and sooner rather than later you would get an out of battery discharge.
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