SureFire Bolt Carrier Group Analysis

AR/M4 AR Technical Discussion
1

Well, I finally got some more information on this design, and I am less impressed.

First off, I thought the cam path was lengthened and the cam angle swallowed out. This is not the case, the cam path is merely widened which introduces potential problems. Below is a diagram of the SureFire cam path superimposed on the MIL-Spec cam path:

You can see all that has been done is move the unlocking cam forward 0.062 inch. This means the cam pin gets to accelerate another 1/32 inch before slamming into the cam path. Further, the cam angle is the same, so the rotational loading on the cam pin is the same.

Then there are other problems.

Because the front point of the cam path was moved forward, it pulled the bolt rings forward in the bolt carrier. The rings get pulled far enough forward they exit the 0.500 bore and fall in the chamfer to the 0.530 bore. This will increase wear on the bolt rings and increase the likelihood of twisted bolt rings.

So, what the reason for the longer cam path to start with? According to the literature it was to increase the length of time from the start of bolt carrier motion to the unlocking of the bolt to ease extraction. So, how does it do in this regard?

So, by taking a high speed camera with a frame rate of 10,000 frame per second (fps), you can film the bolt carrier motion through the ejection port window. By measuring the movement of a mark on the carrier with a frame-by-frame examination of the film you can figure out the time it takes for the bolt carrier to move backwards the 0.325 inch for a MIL-Spec carrier and compare it to the time it takes to move the 0.387 inch for the SureFire carrier. So, 18 rounds were fired in a freshly cleaned AR with a MIL-Spec carrier and filmed. Then the weapon then cleaned again and the bolt carrier group replaced with a SureFire BCG and the test repeated.

The average time to unlock for the MIL-Spec BCG was 0.01081 seconds.

The average time to unlock for the SureFire BCG was 0.01066 seconds.

With an average difference of 0.000144 seconds (0.14 milliseconds, the observed difference was 1 to 2 frames).

I’m having trouble correlating this 0.014 milliseconds to a pressure reduction, as I’m not 100% sure the pressure-time curves I have access to, have enough resolution for this, nor due I have a good enough ignition zero-time to sync the pressure curve to the unlocking.

CONCLUSIONS:

The widened cam path may delay the unlock until the pressure has been reduced by 15%, but I can’t tell, it might be more, it might be less. In any case the unlocking is only delayed less than 1.5%. The most worrying to me is the fact that the gas rings start to leave the 0.500 inch bore. Unfortunately, the length of the 0.500 inch bore cannot be increased due to interference with the bolt when the bolt is fully retracted.

Then there is the increased carrier velocity and impact load on the cam pin due to the longer dwell track. The cam path should have been designed as below, this would have reduced the cam pin loading.

It would have delayed unlocking the same as their design, but also would have reduce the energy loss due to bolt rotation. (And, would simplify manuafture.)

2

Am I reading this right? The regular carrier unlocked later?

3

Wouldn’t an increasing radius reduce cam pin load more than a reduced angle?

4

You were reading it right, but the numbers were reversed.

Corrected original post.

5

What radius? The one at the top of the cam path?

That radius has been increase slightly. If that radius is increased any more, then the cam angle must be reduced. As shown the cam pin load remains more, or less, the same but the bolt is rotated over a longer period of time, so the bolt carrier retains more velocity. This will aid extraction.

6

Not surprised.

Thanks for the informative post!

7

Outstanding post, Lysander. Fascinating. This needs to be pinned, IMHO.

Any appetite for the same treatment on an LMT Enhanced Carrier?

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8

Is this the carrier that Jim Sullivan designed for his Arms West constant recoil upper or something? The thing is basically vaporware so I can’t say Im disappointed.

9

Yes. Would a larger radius there reduce cam pin load even if the final angle is not reduced? Because pressure/bolt thrust is higher at the beginning of the cam pin path and lowers towards the end. Or am I overestimating the pressure drop in that time?

10

Increasing the radius will decrease the pin load slightly. Look at the image below That is a force diagram for the pin loading. “B” is the force required to rotate the bolt, and will be a constant due to friction and inertia of the bolt. “A” is the force only due to friction between the cam pin and the unlocking surface*. Since the force from the locking surface is always perpendicular to the locking surface as the radius increases (or the cam angle decreases) the “A” will decrease.

Now, instead of just that little short radius, you have the entire cam length to work with. You will save a little bit by increasing the initial radius, but you will save more by leaving the radius alone, but shallowing out the entire cam path. Look below and assume the red circle cannot move in any direction but up and down on the page, which cam path will use the least energy moving the red circle down?.

  • While the gas pressure is generating lots of force, the only way it can get to the pin is through the contact area between the unlocking surface and the pin. Since the bolt is free to rotate, the only “downward” force is that overcoming the bolt resisting rotation, and the only “rearward” force is the sliding friction between the pin and unlocking surface. The major loads on the cam pin are the loads generated when it stops after unlocking.
11

Where is the highest stress on the bolt from the cam pin? I guess that’s really the more important question because I’ve seen a bolt broken by the cam pin, but never seen a broken cam pin.

12

This is all really good information lysander. Thank you.

Joe Mamma

13

The highest stress in the bolt from the cam pin is where there is the least material, the two webs either side of the cam pin hole, even more specifically at the top of those webs. The cam pin does two things, it places a longitudinal load on the bolt, stretching it, and it tries to rotate, placing a bending load at the webs.

14

Where in the unlocking cycle is the highest stress on the bolt at the cam pin hole?

This seems like the low hanging fruit of adjusting the cam pin slot to me. Actually reducing a known failure point.

15

Apparently CAG (Delta, or whatever they go by now) had supposedly adopted this BCG and the Geissele SD-ish platform for acquisition a year or so back to replace their HK416’s. Not sure how far along in the pipeline they are though. The BCG itself is unobtanium vaporware, and has been for quite some time…unless you want to drop $800+ for one on Gunbroker.

I would assume our tip-of-the-spear folks would have done testing, especially given that it was a BCG from one maker and the carbine from another; I have to admit that this was an unusual combo given the Geissele REBC that usually comes with the gun.

Any thoughts Lysander?

16

If I had to guess, likely incorrectly, I would wager it is the moment the cam pin bottoms out in the forward extent of the cam pin track (initiation of rearward bolt movement, after rotational unlocking).

But, that’s literally just a guess. Will defer to Lysander or Constructor.

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17

That is where the highest loads are on the cam pin and the hole it is in. The entrance to the cam angle is not a very highly loaded area, as the only loads are from sliding friction of the cam pin on the side of the path, the friction of the bolt in the carrier and the inertia of the bolt. And in order to make any large changes to that radius, you have to increase the actual cam angle, which start to negatively effect the bolt carrier velocity.

I just thought as the only time anyone actually altered the carrier geometry (to my knowledge), i thought it would be interesting to examine. There probably is some merit to the idea.

18

How does the cam path compare to the one in LMT enhanced carrier? Does it have the same issue with regards to the gas rings?

The LMT carrier allows the bolt to travel farther forward. In fact, there is a protruding collar on the front of the carrier to retain the extractor pin as captive beyond the normal travel of the bolt.

19

Essentially doesn’t matter as they are unobtanium.

20

FCD has complete E-BCGs in stock right now. I bought one a few months ago too. They haven’t been that hard to find.

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