Long stroke SureFire Carrier

[Shootin' Bruin]

Surefire claims 3 things with their OBC:

1 - Extraction at a lower chamber pressure due to the revised cam path. Purportedly this aids in extraction reliability as well as reduces ejection port pop.

2 - Better feeding reliability as a result of increased cycle distance and a counterweight inside the carrier.

3 - The elimination of bolt bounce, also a result of said counterweight.

Honestly, there is nothing their counterweight does that a properly selected buffer doesn't do already. As to the other things, assuming Surefire's claims are true, it is still arguable that they are unnecessary for a perfectly serviceable AR. There are literally millions of examples of the AR family of weapons out there that run perfectly fine without these features.

Under very specific use cases (such as a select fire short barreled AR that must be able to cycle properly with and without a suppressor without the aid of an adjustable gas block) perhaps Surefire's BCG does make for a more controllable, shootable, and reliable AR. Does this mean every AR needs one? Probably not. Does this mean you should or should not get one for your AR? Only you can answer that.

I don't feel like reading all 22 pages and am not really into tech jargon but am curious as to what problem this is a solution to? The current day AR/M4 has been refined and proven to the point that now the search for minuscule performance increases are worth spending money on? I also have a strong dislike of the introduction of non standard parts that have the potential of destroying a rifle, bad enough that the A5 introduced a third buffer length, do we really gain by having four with one only usable with a special BCG?

[Shootin' Bruin]

Hi Clint, what are your opinions on cutting down a mid-length gas barrel to 11" for full time suppressed use? Assuming proper gas port sizes for both, would there be any measurable benefit between an 11" mid and an 11" carbine gas system?

[Tokarev]

2 - Better feeding reliability as a result of increased cycle distance...

*Honestly, there is nothing their counterweight does that a properly selected buffer doesn't do already. *

Except the first part of bullet #2 you noted above.


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[Shootin' Bruin]

A standard AR buffer cannot increase cycle distance, but neither can the counterweight on its own. My only point is that the function of slowing forward carrier travel that Surefire claims for its in carrier counterweight can also be performed by a properly selected buffer. Whether the extra weight is in the buffer body or the carrier makes no difference to the function of the gun.

Except the first part of bullet #2 you noted above.


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[SethB]

Honestly, there is nothing their counterweight does that a properly selected buffer doesn't do already. As to the other things, assuming Surefire's claims are true, it is still arguable that they are unnecessary for a perfectly serviceable AR. There are literally millions of examples of the AR family of weapons out there that run perfectly fine without these features.

If the ROF that Sullivan claimed can be achieved by the OBC then it's worth it just to reduce ammunition consumption and heat on auto. The A5 buffer reduces ROF by about 12% over the conventional carbine buffer. Nice, but not enough.

Using buffer weight alone to slow rate of fire isn't necessarily going to be reliable.

And heat, more than anything, is the enemy.

[Clint]

I think that's going too far. The MID will have a large portion of the total gas impulse in the blow down phase where it will be more sensitive to temperature and suppressor characteristics. Something in between is a better balance, IMO.

Hi Clint, what are your opinions on cutting down a mid-length gas barrel to 11" for full time suppressed use? Assuming proper gas port sizes for both, would there be any measurable benefit between an 11" mid and an 11" carbine gas system?

[Shootin' Bruin]

I'm not disagreeing with you. But what I am saying is that the reduction in ROF, elimination of bolt bounce, and enhancement of feeding reliability that is brought about by the counterweight ALONE can be matched by a properly selected buffer. Obviously the OBC as a whole is more than just the counterweight. I will also say that if heat and ammunition consumption were the only concerns, then shooting the same number of rounds in a given time frame generates the same amount of heat and consumes the same amount of ammo regardless of the gun's cyclic rate. Surefire mainly emphasizes relability and controllability as the benefits of their OBC.

I've actually experimented with increased cycle distance 7 years ago. The strike face of the bolt carrier that Adams Arms uses in its piston systems is shorter than the gas key in a standard DI carrier. Using an A5 length buffer tube with a standard 3.25" buffer and the equivalent of 3 quarters to act as spacers in the rear of the tube resulted in a cycle distance of ~4.4" vs. the ~3.9" of a standard AR. With an H3 buffer and an AR-10 spring, measured ROF was 530-550 rpm. With Bill Alexander's special Enidine buffer, ROF was 440-480. It was so slow that a well practiced shooter could fire a standard AR on semi-auto at a faster cadence than this thing cycled on full. It's wonderfully controllable, even more controllable than an MP5. It can hold a 90 round mag dump in the A zone of an IPSC target at 15 yards. More importantly, with 2 round bursts dispersion between the 2 rounds is only 2-4" at that range. I can post video if someone could point me towards how one embeds video into a post.

I tried shortening the gas key in a standard DI BCG as well but no amount of staking and Rocksett would stop the one screw from coming loose after a few hundred rounds. I even tried welding the screws into the gas key as you can see in the second picture but even that didn't prevent leakage after awhile. I'm sure Surefire's done their homework for this and am looking forward to seeing someone test it. Voodoo and Salient Arms also offer carriers that have integral gas keys so I'm curious to why Surefire didn't decide to go that route.

Here is my initial attempt. Yes, the carrier is lightened; this was for a competition rifle.

72791009_2636706559706479_2146817489982455808_n.jpg

Second attempt with welded fasteners. The stem of the rear screw is still in the carrier. It is welded to the rear of the gas key which itself was shaved down to fit inside the buffer tube:

22894134_1670924286292835_1054232933546386722_n.jpg

If the ROF that Sullivan claimed can be achieved by the OBC then it's worth it just to reduce ammunition consumption and heat on auto. The A5 buffer reduces ROF by about 12% over the conventional carbine buffer. Nice, but not enough.

Using buffer weight alone to slow rate of fire isn't necessarily going to be reliable.

And heat, more than anything, is the enemy.

Thanks for the advice Clint.

I think that's going too far. The MID will have a large portion of the total gas impulse in the blow down phase where it will be more sensitive to temperature and suppressor characteristics. Something in between is a better balance, IMO.

[lysander]

Relocating the gas port down stream adds delay two ways.

The port is further down the barrel so it starts pressurization later and the gas tube is longer, which increases the time to transmit the initial pressure pulse.

These two curves are copies of actual measured carrier piston cavity pressures from a test the Army did back in 2009.

T = 0 is the ignition of the primer. You can see the time delay to peak pressure is 0.5 ms in the longer gas system. You may also note a small discontinuity in the decay slope of the carbine pressure curve. That occurs at the same time as the cam pin first enters the helical portion of the cam track, and I believe is due to the resistance of the carrier trying to turn the bolt against the residual chamber pressure.


The following curves are models based on the above pressure curves.

Here is the velocity history from T = 0 to the end of the piston stroke for three different buffer weights and the rifle configuration.


Here is the carrier travel history. The three horizontal lines are the three events noted, start of unlocking, end of unlocking and end of piston stroke. Note the start of unlocking begins at t = .00160, the same time the discontinuity occurs in the pressure curve.

[StainlessSlide]

Lysander,

Thank you for posting the pressure graph and the two integrals. Did you use an Excel integration routine?

I have dreams of high-speed cameras, to get confirmation of these results.

In #203 you wrote that an time difference of .5ms was necessary to match carbine and rifle. On your graph, it looks like the unlocking time difference between the H1 carbine and the rifle is about .35ms, if that is linearly extrapolated back to the standard carbine buffer we get a difference of about .38ms.

The 1.6 oz difference between the H1 and H3 buffer gives a delay of .05ms (your number), linearly extrapolating this to the 3.8oz difference between the standard and the A5H4 buffer we get a delay of 1.2ms. The 5.5oz difference between the standard and XH buffer from Heavy Buffers gives a delay of .17ms.

Switching to these two buffers from the standard gets us 31% and 45% of the way along the standard carbine/rifle gap at unlocking.

At the beginning of extraction (which should take place at the piston max travel, or thereabouts, since the cam is at the end of the track) the H1/rifle gap looks like .38ms, which extrapolates to maybe .44ms with the standard buffer. The H1/H3 gap here looks like .13ms. An analysis similar to the above gives .31ms delay for the A5H4 buffer (70% of the carbine/rifle gap), and a .45ms delay for the XH buffer (102% of the carbine/rifle delay).

According to this the time benefit of a buffer change can give a substantial proportion of the carbine/rifle benefit at unlocking, and a bigger proportion at extraction.

The hash marks on your graph are a bit small for these eyes, you could do the above analysis with the precise numbers. Also, the time gaps from the H1-H3 buffers at extraction look a bit nonlinear, and I am using linear extrapolation.

Speaking of error analysis, I don't see any in my cursory reading of the papers you referenced.

[StainlessSlide]

The AR system was designed around a rifle length gas tube, so that is where it will work best.

Sullivan says the design was quite hurried, I would not make the assumption above. Recall the Edgewater buffer.

Thank you, Bruin for the fun video.