I am trying to understand why it is actually in the design.  I get that it functions with the bolt rotation but beyond that...??? Please enlighten me.

Why not rounded like POF's roller cam pin?  Thoughts on these?

It translates the rearward linear movement of the carrier into rotational movement at the bolt.

The top part is square to aid in indexing the firing pin hole, I imagine.

I never really thought about it, but here goes...

I have to assume the cam pin 1) keeps the firing pin inline, 2) keeps the bolt in the BCG and 3) allows the bolt to rotate/move forward and back by catching the protrusion on the left side of the upper.

Once again, this is just a shot in the dark.  I'd like to know now.  If someone with more experience can shed some light, please do so.

Without the cam pin:

-If the bolt were locked and the gun was fired, the bolt would never unlock from the barrel extension and would be left stuck to the case head in the chamber and never extract the empty case.

If the bolt carrier were open and then closed on a live round (assuming the bolt aligned with the barrel extension without the cam pin), the bolt would never rotate into the locked position when going into battery and your gun would violently disassemble itself when you pulled the trigger.

as the cam moves forward there is a notch in the bcg that "cams it" to the right, locking the bolt.



I.e. it locks the bolt through camming action

The protrusion on the left of the upper isn't supposed to catch anything, it's just clearance so the bolt can rotate into the locked position in the barrel extension.
The slot in the upper below the charging handle keeps the cam pin/bolt aligned so it will not try to lock from the resistance of stripping a round from the mag before entering the extension.

As the bolt is moving forward and stripping a round from the mag, the bolt is being pushed back into the carrier to the locked position , the only thing that stops this from happening is the cam pin riding in its slot in the upper, the bearing surface for this is the flat side of the cam pin. If it were round, only a small section of the cam pin would be a bearing surface, a smaller surface that would wear faster then the larger flat surface we have now, causing the bolt to turn slightly and impact the lugs on the barrel extension.  The roller type cam pins alleviate this, but at the expense of more moving parts.  Moving parts complicate things, and are just more sources of failure.

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The reason POF made the roller cam pin was because in piston rifles that use the standard bolt and remove the gas rings were havin issues.

See without the gas rings there was no tension on the bolt, so when it would go to pick up a round from the magazine the cam pin would then rub against the upper. Over time this was leading to excessive premature wear on the upper, so POF made the roller cam pin to combat this, since it rolls it no longer ground down the upper.

This is something not needed in the standard DI system since the gas rings give you all the tension you need when stripping a new round.

Already...well now I know.  Thanks!

Interesting; I don't have much experience with external piston ARs but this just goes to confirm my opinion that they're more trouble than they're worth.

I do t buy that for a second, the gas rings dont hold that much tension. Not enough to keep the bolt from locking while feeding.

The need for the roller cam pin has more to do with the fact that there is nothing that actively unlocks the bolt, it's just the movement of the BCG in a piston set up that unlocks the bolt, causing issues. In a DI gun the bolt is actively opened, it's not jus the movement of the carrier, it's the forces brought by the gas acting to push the bolt forward and the BCG rearward, that opens the bolt.

In a piston gun the bolt is only turned enough to unlock, not to it's extreme forward unlocked position.  Because on a piston gun the bolt only turns enough to unlock the bolt, the cam pin may not be exactly where it was designed to be, thus causing problems.  With DI the bolt is all the way forward turned a much as mechanically possible, the cam pin is vertical.

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Then i guess its just one big coincidence that on a DI you just get finish wear, but in a piston that uses the standard bolt(minus gas rings) they get actual damage

I didnt say every piston has this issue just the ones that use a standard bolt minus the gas rings.

Besides i was told this by someone who would know

Sorry, I edited to say why it does this, pistons cause problem with the cam pin, DI doesn't,  it has nothing to do with lack of gas rings in a piston gun.  Piston guns will benefit from roller pins, I agree with that assertion.  Has nothing to do with gas rings though.

Take a brand new bolt, with brand new tight gas rings an push it at speed into the back of rounds In a magazine, and tell me if the bolt collapses.


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Oh i know the bolt will collapse. The difference is the bolt will collapse with less force, this will keep it from jamming and rubbing into the receiver.

On DI when the bolt collapses it does it slower and with less force which leads to just finish wear.
On a piston the bolt slams shut jamming the cam pin into the side this cause excessive wear after a while.

Fair enough, I'll buy that. How effective the rings are at this is up for another discussion.  I still think my assertion holds true as well to a certian point.  I've seen "more then normal on a DI gun" damage to the back corner of the cam pin relief in piston guns that made me assume that's the reason for the roller cam.

Makes me wonder if high round count DI guns with loose bolts are chewing them selves up


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Well certainly learned some new things and some things to ponder.