Long stroke SureFire Carrier

[lysander]

Increasing free travel in the cam path also results in a gun less sensitive to different types of ammo I believe. One could adjust the gas port to provide complete cycling with low pressure ammo while still maintaining reliable extraction with high pressure ammo by virtue of the delayed unlocking. The Lewis machine gun has extremely long free travel built into its cam path, more than an inch. Could this be because military ammunition at the time was far less consistent than it was when the M16 was developed?

Lysander, that is a sweet gun! Did you design it yourself?

Yes and no.

Increasing the free travel delays unlocking, but you also get higher bolt velocity for the same amount of gas. Higher bolt velocity brings with it its own set of extraction and cycling issues. So, an increase in free run before unlocking may make the system more sensitive to port pressures outside the design assumptions.

The whole system is, well.... a system. Changing one parameter of the system cannot fix all possible problems. The AR system was designed around a rifle length gas tube, so that is where it will work best. Changing the length of the gas tube introduces problems that can be addressed in various ways, and each proposed "fix" will introduce other limitations, or problems. Sometimes the new limitations or problems are less severe or easier to live with, and sometimes they are just moving the problem from "here" to "there".

This problem was described once to me with "The balloon in a box" analogy. Image the AR-15 with a rifle length gas system is a balloon in a box, both of which have the exact same volume. The balloon can be placed in the box and the lid closed and it fits. Now, the carbine length gas system is a box with a slightly smaller volume, but the balloon is still the same volume as before. Now, when the balloon is put in the box and you try to close the lid, part of the balloon will pop out of the box. You can push the part of the balloon that sticks out in, but it will just pop out somewhere else. The trick to this is getting the balloon to stick out somewhere where it bothers you the least. but just understand that unless the entire system, the balloon and the box together, are re-designed, you're going to have something that is not optimal everywhere.

There is a reason the the buffer and the gas key are shorter, and it is an out growth of the revised cam track. If you make the cam track longer the distance the bolt protrudes from the carrier when unlocked is longer by the same amount. For example, the standard AR bolt face is 0.778" from the face of the bolt carrier, if you make the cam track longer by 0.150" the bolt face will now stick out 0.928". The face of the bolt has to move rearward at least 2.8" to get behind the bolt catch, and probably another 0.200" to ensure the catch has time to rise to engage. This means the bolt carrier minimum travel has increased an additional 0.150" to get the face of the bolt back to where it needs to be... It might be more than that if the longer protrusion of the bolt requires repositioning the cam track to keep sufficient 'wheelbase' on the bolt support rings.

As to the Lewis, FG-42, and M60 and why they have such long free travel. Well, first off, they don't have much free travel at all, maybe a 1/10 of and inch maximum. The cam helix is just very shallow over the first half of the run, the illusion is multiplied by the very long actuator stud on the operating rod, making the entire cam look long. But, that aside, remember when we said the longer the free run, the higher the carrier velocity? A higher carrier velocity, means the carrier has more momentum. If the bolt assembly mass is large compared to the carrier/op-rod mass, more velocity will be lost when the carrier/op-rod has to pick up that mass and bring it rearward. The mass of the Lewis, FG-42 and M60 bolt assembly is fairly substantial compared to the op-rod mass, and the Lewis and M60 need a lot of energy (velocity) to run the feed systems. Also, these weapons have helical locking surfaces, which means as the bolt unlock the bolt also moves rearward slightly, initiating slow extraction during unlocking, the very shallow cam helix is done to maximize the mechanical advantage of the actuator stud in the cam track.

And, yes.

[SethB]

There isn't a single component on a modern rifle that is identical to the AR15 that LeMay used to kill watermelons.

The original design wasn't perfect. The Edgewater buffer is proof of that.

I got a kick out of the fact that the M16A4 is more reliable with an A5 buffer than it was unmodified.

[mack7.62]

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?

[Sry0fcr]

The nerd talk makes me hard. Seeing the math explain what I've understood is of great benefit.

Just curious, what your opinions on how a PRI Fat Boy would affect cycling on a carbine gassed gun. Seemed like they were on to something way back when...

[1168]

The nerd talk makes me hard. Seeing the math explain what I've understood is of great benefit.

Just curious, what your opinions on how a PRI Fat Boy would affect cycling on a carbine gassed gun. Seemed like they were on to something way back when...

I’ve also been enjoying the nerdy bits of this thread, and its given me plenty to think about.

About that PRI gas tube: I remember like 15 years ago, there was a spiral gas tube that people were using on pistols. I wonder if it was, or is, useful.

[Sry0fcr]

I’ve also been enjoying the nerdy bits of this thread, and its given me plenty to think about.

About that PRI gas tube: I remember like 15 years ago, there was a spiral gas tube that people were using on pistols. I wonder if it was, or is, useful.

I think the concept probably had more merit than the execution.

[lysander]

Increasing the internal volume of the gas system by increasing diameter of the gas tube is another way to slow down the time to unlock. How much delay you get depends on how big the volume is. The pig-tail gas tube was just a different way to accomplish this. But once again it starts to introduce other things, thermal losses to the gas, and possible fatigue issues with tube flex from inflating and deflating a pig-tail, and with the fat tube, possible precipitation of combustion products inside the tube due to reduced velocity, pressure and temperature.

Personally, the way I see it, go with the longest gas system you can for the given barrel length, and if you have something carbine length or shorter, learn to live with a 10,000 to 12,000 round average bolt life.

[Sry0fcr]

Increasing the internal volume of the gas system by increasing diameter of the gas tube is another way to slow down the time to unlock. How much delay you get depends on how big the volume is. The pig-tail gas tube was just a different way to accomplish this. But once again it starts to introduce other things, thermal losses to the gas, and possible fatigue issues with tube flex from inflating and deflating a pig-tail, and with the fat tube, possible precipitation of combustion products inside the tube due to reduced velocity, pressure and temperature.

Personally, the way I see it, go with the longest gas system you can for the given barrel length, and if you have something carbine length or shorter, learn to live with a 10,000 to 12,000 round average bolt life.

Heh, seems I had similar thoughts about 10 years ago. Your last sentence is sage advice though I'd add that you should run the heaviest buffer you can get away with and keep up with your extractor and springs (extractor and action).

[Shootin' Bruin]

Yes and no.

Increasing the free travel delays unlocking, but you also get higher bolt velocity for the same amount of gas. Higher bolt velocity brings with it its own set of extraction and cycling issues. So, an increase in free run before unlocking may make the system more sensitive to port pressures outside the design assumptions.

Is the increase in velocity sufficient to cancel out the delay in unlocking the bolt would have had if velocity were the same? If the free travel before the start of unlocking were doubled for example, wouldn't carrier velocity also have to double as well for no added delay to take place? Or are the extraction and cycling issues you're referring to pertain to something other than chamber pressure at the moment of extraction?

There is a reason the the buffer and the gas key are shorter, and it is an out growth of the revised cam track. If you make the cam track longer the distance the bolt protrudes from the carrier when unlocked is longer by the same amount. For example, the standard AR bolt face is 0.778" from the face of the bolt carrier, if you make the cam track longer by 0.150" the bolt face will now stick out 0.928". The face of the bolt has to move rearward at least 2.8" to get behind the bolt catch, and probably another 0.200" to ensure the catch has time to rise to engage. This means the bolt carrier minimum travel has increased an additional 0.150" to get the face of the bolt back to where it needs to be... It might be more than that if the longer protrusion of the bolt requires repositioning the cam track to keep sufficient 'wheelbase' on the bolt support rings.

While I do not have the Surefire OBC on hand, I do have an LMT Enhanced BCG which also has an extended cam track and can tell you that it will clear the bolt catch in any standard AR with no changes to the buffer. Perhaps the SF OBC's track is even longer, but I do know from both InRange's interviews with OBC designer Jim Sullivan and from what Barry Dueck said in promotional material that the primary purpose of the shortened gas key and buffer are to increase cycle distance in order to both lower the rate of fire and to make the system more tolerant of weak magazine springs and clogged magazines. In addition, Sullivan told InRange that he is attempting to replicate as much as possible the constant recoil system of the Ultimax LMG within the confines of the AR receiver. In a SHOT Show video from a couple years ago, Dueck also mentioned that the initially planned to offer the OBC in both long stroke (what they are offering now) and standard stroke configurations.

In regards to the bolt being properly supported by the carrier despite increased protrusion, LMT addresses this by designing the front of their carrier to extend into the barrel extension. This also prevents the extractor pin from being exposed when the bolt is at full extension. I do not know how Surefire addresses this issue.

As to the Lewis, FG-42, and M60 and why they have such long free travel. Well, first off, they don't have much free travel at all, maybe a 1/10 of and inch maximum. The cam helix is just very shallow over the first half of the run, the illusion is multiplied by the very long actuator stud on the operating rod, making the entire cam look long. But, that aside, remember when we said the longer the free run, the higher the carrier velocity? A higher carrier velocity, means the carrier has more momentum. If the bolt assembly mass is large compared to the carrier/op-rod mass, more velocity will be lost when the carrier/op-rod has to pick up that mass and bring it rearward. The mass of the Lewis, FG-42 and M60 bolt assembly is fairly substantial compared to the op-rod mass, and the Lewis and M60 need a lot of energy (velocity) to run the feed systems. Also, these weapons have helical locking surfaces, which means as the bolt unlock the bolt also moves rearward slightly, initiating slow extraction during unlocking, the very shallow cam helix is done to maximize the mechanical advantage of the actuator stud in the cam track.

I cannot speak to the FG or the M60, but there is a very detailed video on the Lewis Gun that illustrates the long cam track: https://www.youtube.com/watch?v=FlsEmE5pM10

An animated cutaway showing the operating system can be seen 42 minutes into the video. AT 59:55 Othias also mentions that Lewis lengthened the cam track further when he experimented with converting the gun to .30-06.

And, yes.

Sweet. I'm pretty sure the carrier is proprietary, but did you make your own bolt as well or are you using a standard AR bolt and barrel extension?

[Clint]

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.

Personally, the way I see it, go with the longest gas system you can for the given barrel length, and if you have something carbine length or shorter, learn to live with a 10,000 to 12,000 round average bolt life.