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.)