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Posted: Nov. 16 2009, 22:32 ET


Here is a sar-21 rifle:

It uses a ar-15 style lockup, but with a fixed op-rod like the AK.

Now one thing I like about the AK is that it's pretty hard or impossible to put the bolt in wrong, it either goes in right or not at all.

Here is what happens if you're assembling a sar-21 in the dark and forget to include the cam pin!



Luckily the shooter was not injured, but it's a big reminder to all of us to try and keep safety in mind in our builds.  

Also it reminds us that .223 is not appropriate for straight blowback, even accidentally!


Posted: Nov. 17 2009, 11:23 ET


Looks like the Kevlar cheekpiece probably did it's job and saved the shooter.

Posted: Nov. 17 2009, 11:26 ET


Quote (geodkyt @ Nov. 17 2009,06:23)
Looks like the Kevlar cheekpiece probably did it's job and saved the shooter.

I thought that myself, good thing he did't shoot it in his off hand!

I'm wondering couldn't you design a firing pin block into the carrier that would stop the FP if the cam pin was missing or bolt wasn't locked up? I know the garand's FP had a little tab that kept it from going all the way forward unless the bolt was rotated the right way.

Notice the oval slot cut in the side of the barrel lockup area? You guys think that is to vent gas out the left side while it's still contained by the barrel extension?


Posted: Nov. 17 2009, 16:41 ET


I've NEVER heard of a bolt cam pin being left out -- and I used to deal with Basic trainees.

The closest I've run into is the guy who gets up and says -- "Hey, I've got some extras, damnit!"

Of course, we always taught the kids to "snap" the bolt group with a flick to make sure the bolt was fully extended, so it would disassemble properly.  If you do that without a cam pin, you'll not be having a Ka-BOOM with THAT bolt group.  Not until you find your bolt in the weeds about 15 feet away. . . :p  (Saw this once, only it was across the company area the bolt went skittering.)

NOTE -- the absence was noted WELL before moving out to go and "do stuff" with the weapon.

We even taught them to use their helmet or hat to hold ALL the little parts until they had reassembled function checked the rifle.  Went so far as to point out that if, God forbid, they start taking fire while their gun was field stripped, they could move to cover with all their parts, and finish assembly there.  (Especially if using the patrol cap -- you can fold it up and shove it into your pocket in a second, and have perfect reassurance that your parts will still be there later.)

Heck, this troop didn't even apparently tilt his carrier group forward -- or the bolt would have fallen out.  If I was investigating this, I would seriously consider self-sabotage.

I don't see how -- other than by adding something like the eternally condemned "magazine disconnect" of the GP35 or the equally loved Series 80 "Firing Pin Block" -- you can institute something that will block the FP if a part is MISSING.  And such a small setup internal to the bolt group is likely to be a reliability hazard.  

It's not hard to block the FP so that it cannot go home unless the operating parts are in the correct alignment, but this?


Posted: Nov. 17 2009, 17:14 ET


Quote (geodkyt @ Nov. 17 2009,11:41)
I've NEVER heard of a bolt cam pin being left out -- and I used to deal with Basic trainees.

The closest I've run into is the guy who gets up and says -- "Hey, I've got some extras, damnit!"

Of course, we always taught the kids to "snap" the bolt group with a flick to make sure the bolt was fully extended, so it would disassemble properly.  If you do that without a cam pin, you'll not be having a Ka-BOOM with THAT bolt group.  Not until you find your bolt in the weeds about 15 feet away. . . :p  (Saw this once, only it was across the company area the bolt went skittering.)

NOTE -- the absence was noted WELL before moving out to go and "do stuff" with the weapon.

We even taught them to use their helmet or hat to hold ALL the little parts until they had reassembled function checked the rifle.  Went so far as to point out that if, God forbid, they start taking fire while their gun was field stripped, they could move to cover with all their parts, and finish assembly there.  (Especially if using the patrol cap -- you can fold it up and shove it into your pocket in a second, and have perfect reassurance that your parts will still be there later.)

Heck, this troop didn't even apparently tilt his carrier group forward -- or the bolt would have fallen out.  If I was investigating this, I would seriously consider self-sabotage.

I don't see how -- other than by adding something like the eternally condemned "magazine disconnect" of the GP35 or the equally loved Series 80 "Firing Pin Block" -- you can institute something that will block the FP if a part is MISSING.  And such a small setup internal to the bolt group is likely to be a reliability hazard.  

It's not hard to block the FP so that it cannot go home unless the operating parts are in the correct alignment, but this?

But the reason it went ba-boom is cause the cam pin didn't lock the bolt, so if the design had a 'no FP unless bolt is locked' then it should pre-vent this shouldn't it?

I mean even with NO cam pin, if the bolt was locked it shouldn't ka-boom?

Posted: Nov. 17 2009, 22:36 ET


Quote (buster charlie @ Nov. 17 2009,17:14)
But the reason it went ba-boom is cause the cam pin didn't lock the bolt, so if the design had a 'no FP unless bolt is locked' then it should pre-vent this shouldn't it?

I mean even with NO cam pin, if the bolt was locked it shouldn't ka-boom?

If I recall correctly, the Mini-14 has a tail on the firing pin that prevents it from moving forward unless the bolt is locked, which lines up the firing pin with a slot in the receiver.  

Another way to do it might be to have a projection on the rear of the bolt face that prevents the hammer from hitting the firing pin unless the bolt rotates far enough to move the projection out of the way.

Another possibility is to have a spring-loaded firing pin that is retained by the cam pin.  If the cam pin isn't installed, the firing pin simply won't stay in its channel.

Posted: Nov. 17 2009, 23:09 ET


Quote (Golovko @ Nov. 17 2009,17:36)
Quote (buster charlie @ Nov. 17 2009,17:14)
But the reason it went ba-boom is cause the cam pin didn't lock the bolt, so if the design had a 'no FP unless bolt is locked' then it should pre-vent this shouldn't it?

I mean even with NO cam pin, if the bolt was locked it shouldn't ka-boom?

If I recall correctly, the Mini-14 has a tail on the firing pin that prevents it from moving forward unless the bolt is locked, which lines up the firing pin with a slot in the receiver.  

Another way to do it might be to have a projection on the rear of the bolt face that prevents the hammer from hitting the firing pin unless the bolt rotates far enough to move the projection out of the way.

Another possibility is to have a spring-loaded firing pin that is retained by the cam pin.  If the cam pin isn't installed, the firing pin simply won't stay in its channel.

I like the spring loaded firing pin idea because it's pretty simple, except then you just have to deal with what if someone looses the spring and the cam pin! ;)

Posted: Nov. 18 2009, 09:48 ET


I think all of Golovko's ideas are eminently practical.

Eventually, you get to a point where you just have to say, "Look, stupid, if you leave enough parts out of the gun when you put it back together, it might blow up."

The only problem with the idea of using a cam in the bolt that will only allow the FP to travel forward if the bolt is locked is that we still haven't addressed the issue of a bolt not actually attached to the bolt carrier.  If there is ANY incline on the locking lugs (say, such as to provide primary extraction during unlocking or if there is wear present), then once the bolt carrier rushes to the rear WITHOUT the "locked" bolt, the only thing keeping that bolt from unlocking is the friction of the lugs -- as opposed to being mechanically held in the proper position by the cam and bolt carrier.

Same problem -- premature unlocking.  Even though it was "locked" when the firing pin hit the primer.  Basically, the gun would be an delayed blowback in function if you left the cam pin out and the bolt managed to rotate into the proper "locked" position before firing.

Also -- you'll need to incorporate a resetting function in the firing pin, or when all the parts are properly present, the bolt can lock up and jam trying to unlock with the firing pin in the forward position.  So you'll really want a spring loaded firing pin that can ALMOST reach the primer if the bolt isn't fully locked.  Not insurmountable.  But it requires the same amount of parts as Golovko's proposed "cam pin retained firing pin".

Having that cam pin retained firing pin AND the bolt that blocks the FP from quite reaching the primer unless the bolt is locked seems about as stupid proof as you can realistically get on a rifle of reasonable reliability.


Posted: Nov. 18 2009, 10:38 ET


Quote (geodkyt @ Nov. 18 2009,04:48)
Having that cam pin retained firing pin AND the bolt that blocks the FP from quite reaching the primer unless the bolt is locked seems about as stupid proof as you can realistically get on a rifle of reasonable reliability.

I think now I figured out why Kalashnikov designed the AK with a fixed cam pin and semi-permanent firing pin on his bolt! Soviet Conscripts are just too good at being stupid. ;)

In all seriousness all of these are good points, you're right you can only hand hold so much but I was just surprised such a popular bolt design had this rather spectacular failure due to a missing part.

Posted: Nov. 18 2009, 13:13 ET


Quote (geodkyt @ Nov. 17 2009,16:41)
I've NEVER heard of a bolt cam pin being left out -- and I used to deal with Basic trainees.

ROTFLMAO  :lolup:  :lolup:

Posted: Nov. 19 2009, 11:45 ET


Forgetting to insert the bolt cam is not a problem if it is solid, milled right onto the bolt, like the AK.

I've never been a fan of the AR-15 style bolt lockup.  Overcomplicated, too many parts, with no pay-off.  The only argument for it is that the bolt has a shorter rotating distance, to which I can only say "so what".

At least the complicated HK G3 has nice features, for example the firing pin can only fire if the bolt is locked.  If you forget any parts, it can not be assembled in a dangerous configuration by accident (you really have to try hard).

Posted: Nov. 19 2009, 12:57 ET


Quote (zastrakara @ Nov. 19 2009,06:45)
Forgetting to insert the bolt cam is not a problem if it is solid, milled right onto the bolt, like the AK.

I've never been a fan of the AR-15 style bolt lockup.  Overcomplicated, too many parts, with no pay-off.  The only argument for it is that the bolt has a shorter rotating distance, to which I can only say "so what".

At least the complicated HK G3 has nice features, for example the firing pin can only fire if the bolt is locked.  If you forget any parts, it can not be assembled in a dangerous configuration by accident (you really have to try hard).

You raise an interesting point with the G3.

Why couldn't you apply the same feature to a non roller locking gun?

For example, there is a non-locking roller on the rear of the bolt carrier that can slide perpendicular to the firing pin. It retains the firing pin at the rear, until the bolt is all the way forward and a cavity in the receiver wall allows the Firing pin to cam the roller into the cavity, if the bolt carrier is not all the way forward it'll not allow the FP to move.  

if you want it linked to barrel rotation, why not have a spring loaded rod that cams off the back of the bolt itself?

In other words, we know the gun is not locked unless it rotates into a certain position relative to the bolt carrier. What if you machined a 'ratchet' like cam surface on the rear surface of the bolt carrier instead of it being flat.

The spring loaded rod is pushed backwards  buy the rear of the bolt, and it rides in a hole that intersects with your firing pin locking roller/ball bearing. In this position the FP cannot move even if struck by a hammer.

When the gun locks, the bolt rotates and the spring loaded rod is allowed to travel forward under spring pressure because now it's aligned with a low spot on the rear of the bolt, this allows the FP to cam the roller out of the way.

As the bolt rotates to unlock during the recoil, it cams the rod backward via a ramped surface on the back of the bolt, and that forces the roller into the FP, which cams the FP back and locks it.

You could also possibly have the gun's safety trigger this also...

Posted: Nov. 20 2009, 03:05 ET


Such a feature has already been achieved without needing additional parts.  The tail on an AK bolt carrier prevents the hammer from being able to hit the firing pin until the bolt has locked.  My idea involving a protrusion on the rear of the bolt performs a similar purpose.  And the firing pin tail I mentioned before on the Mini-14 performs the same purpose.  In fact, on the Mini-14, not only does it prevent the firing pin from moving forward until the bolt it locked, it also helps prevent slam-fires.  If the tail of the firing pin isn't aligned with the slot in the receiver, the firing pin is forced to the rear when the tail contacts the receiver.  This neutralizes all forward momentum the firing pin had, and can help free a stuck firing pin.  

All of these ideas work without introducing any additional parts to the firearm.  I'll bet that ordinarily the SAR-21 can't fire unless the bolt is locked, but of course with a missing cam pin, the bolt could move rearward as if locked without actually rotating to a locked position.  The Mini-14 firing pin system would probably be the best one for use on a rifle with a removable cam pin, as the Mini-14 firing pin simply cannot move forward unless the bolt is in the locked position.

Posted: Nov. 20 2009, 16:28 ET


Quote (Golovko @ Nov. 19 2009,22:05)
Such a feature has already been achieved without needing additional parts.  The tail on an AK bolt carrier prevents the hammer from being able to hit the firing pin until the bolt has locked.  My idea involving a protrusion on the rear of the bolt performs a similar purpose.  And the firing pin tail I mentioned before on the Mini-14 performs the same purpose.  In fact, on the Mini-14, not only does it prevent the firing pin from moving forward until the bolt it locked, it also helps prevent slam-fires.  If the tail of the firing pin isn't aligned with the slot in the receiver, the firing pin is forced to the rear when the tail contacts the receiver.  This neutralizes all forward momentum the firing pin had, and can help free a stuck firing pin.  

All of these ideas work without introducing any additional parts to the firearm.  I'll bet that ordinarily the SAR-21 can't fire unless the bolt is locked, but of course with a missing cam pin, the bolt could move rearward as if locked without actually rotating to a locked position.  The Mini-14 firing pin system would probably be the best one for use on a rifle with a removable cam pin, as the Mini-14 firing pin simply cannot move forward unless the bolt is in the locked position.

I know about the tail on the AK, but it doesn't prevent inertial strikes of the FP, so I like the garand system too (which I assume the mini-14 copies) of having a tab to keep it from going forward unless the bolt is lined up.

I also like the point you make about it camming out a stuck firing pin, it seems like a more positive system than relying on a spring.

Posted: Nov. 20 2009, 16:28 ET


Double post.

Posted: Nov. 21 2009, 02:45 ET


Quote (buster charlie @ Nov. 20 2009,16:28)
I know about the tail on the AK, but it doesn't prevent inertial strikes of the FP...

That's true, but without an autosear, no practical design is immune from that.  About the only way I can think of to avoid that problem would be to design the hammer so that if released prematurely, it catches the bolt/carrier and causes a jam.  I've heard that this can happen with some AR-15's, but I don't know if that's by accident or by design.    

Quote
...so I like the garand system too (which I assume the mini-14 copies) of having a tab to keep it from going forward unless the bolt is lined up.

I also like the point you make about it camming out a stuck firing pin, it seems like a more positive system than relying on a spring.

It also has the benefit that if the firing pin is jammed bad enough that it cannot be cammed to the rear, it will also keep the bolt from traveling far enough forward to cause a slam-fire.  The odds of that scenario are pretty slim in the first place, but it's another potential perk for a system that already has other tangible benefits.

Posted: Dec. 9 2009, 18:38 ET


Quote
I know about the tail on the AK, but it doesn't prevent inertial strikes of the FP...


Quote
That's true, but without an autosear, no practical design is immune from that.


The G3 is.  Even without autosear.
Firing pin is held by the bolt carrier, and can not come in contact with the primer until the bolt head is locked.


I've never gotten ahold of the DP28, which uses locking flaps, but from what diagrams and photos I've seen, it does something similar in a simpler way.  The pin is held in rear bolt piece, and the locking flaps block it's forward motion until they are in a fully locked position.  This makes firing out of battery impossible, and prevents slam-fire inertia strikes, unlike designs where the pin is held within the front of the bolt.

To prevent inertia strikes, just get the pin out of the bolt face, hold it in the back, and design from there.

Posted: Dec. 10 2009, 05:12 ET


Quote (zastrakara @ Dec. 09 2009,18:38)
Quote
I know about the tail on the AK, but it doesn't prevent inertial strikes of the FP...


Quote
That's true, but without an autosear, no practical design is immune from that.


The G3 is.  Even without autosear.
Firing pin is held by the bolt carrier, and can not come in contact with the primer until the bolt head is locked.


I've never gotten ahold of the DP28, which uses locking flaps, but from what diagrams and photos I've seen, it does something similar in a simpler way.  The pin is held in rear bolt piece, and the locking flaps block it's forward motion until they are in a fully locked position.  This makes firing out of battery impossible, and prevents slam-fire inertia strikes, unlike designs where the pin is held within the front of the bolt.

To prevent inertia strikes, just get the pin out of the bolt face, hold it in the back, and design from there.

I had to go back and re-read the posts.  For whatever reason, I had it in my head that he was talking about scenarios where the hammer follows the bolt carrier into battery.  :bangin:

You are correct that there are a number of firearm designs that prevent the firing pin from striking due to the force of intertia.  I even cited an example previously.

Posted: Dec. 10 2009, 16:43 ET


Heck, a simple SPRING holding the firing pin back will prevent inertial slamfires.

Wonder of wonders, I even have, out in my garage, a 9mm blowback bolt for a semiauto that uses an AR15 firing pin in EXACTLY that configuration. . .  :)