Long stroke SureFire Carrier

[lysander]

I would tend to agree with this perspective.

The buffer mass should contribute its energy to the spring.

The buffer mass contributes nothing to bolt pickup or case extraction, as that is up to the carrier mass and residual chamber pressure alone.

Exactly.

And, since the energy in the spring is only retarding things during extraction and ejection, ie, not helping pull the case from the chamber, all the energy required to do this must come from the bolt carrier's initial velocity it has gained during the first three-tenths of an inch movement.

A couple further thoughts and questions.

What is the action spring efficiency and how much energy is lost to friction during the full stroke?[1]

Does the mass of the action spring itself contribute in any meaningful way to the calculations?[2]

Does the gas piston deliver constant impulse/momentum, constant energy or something in between?[3]

1. During the full cycle, this is not a negligible amount. However, you can model drag friction as a constant, as friction usually is, and you can model the hammer over-run drag on the hammer-to-carrier geometry and the hammer spring constant.

What I did is construct the model based on the known M4 data, then play with 'correction constants' until the model reacts to match empirical data. Then you can make your desired changes to buffer weight, carrier weight, spring constants, etc and get reasonable results. Unless you make gross changes, like changing the cartridge to .50 BMG, you will stay in the ballpark.

2. Technically, each coil of the spring (except the last two) are moving backwards with some velocity. The coils in contact with the buffer are moving at the speed of the buffer, and all the way in the back, the last two coils are stationary. You can do the calculus and figure the exact amount of energy lost, but that is about like measuring the height of a mountain with a micrometer. The tolerance on the spring constant is around 4 or 5 N/m, that alone will swamp the these losses in this case. If you had a very heavy spring, and/or a very high velocity, like in a car suspension, that energy loss might be a factor.

3. The gas pressure vs time is as shown in the graphs many posts back, from that you can calculate the force vs time, momentum vs time, energy vs time, etc.

Remember, this is a mathematical model of the system, not a 1-to-1 scale model that reacts exactly like the real thing under all possible circumstances. This is not intended to calculate the exact location a spent case will land, plus or minus an inch, the variables are too many and the tolerances of the parts too wide for this level of accuracy. Go back the the first graph I posted, it notes the time to bullet exit. Do you think ever bullet shot from that barrel take the exact same length of time to exit? And how about all the other 14.5 inch barrels out there? No, but on average they will probably take that long.

What you are looking for are trends: I wish to delay unlocking by 1/2 ms, if I increase the buffer weight, do I get and delay in unlock time? Not really, at least no where near what I want. Okay, I need to look somewhere else to get that 1/2 ms delay I want. What happens if I reduce my piston size? What happens if I reduce the gas port diameter? And so forth; from these results I can figure out the best way to get from where I am now, to where I want to be, with the least amount of fabrication and testing.

[lysander]

Question probably best answered by moderators...

Could the extremely interesting technical information that deviated from the title of the original post be separated into a new thread?

I think this thread has shown that the product detailed in the OP has merit:

1. A longer cam track, increasing time to unlock,

2. coupled with a heavier bolt carrier, reducing the minimum velocity requirement, possibly further delaying time to unlock, and

3. a heavy type buffer reducing the actual bolt carrier/buffer velocity.

All pluses.

And, while not discussed in our lengthy discussion of velocity and mass, the longer allowable bolt carrier travel has to be considered a plus as well.

[JediGuy]

Definitely not a knock on the discussion at all. I think it has been interesting enough that I’d hate for it to be “lost” to people, as it goes beyond the title topic.

[Diamondback]

Definitely not a knock on the discussion at all. I think it has been interesting enough that I’d hate for it to be “lost” to people, as it goes beyond the title topic.

Frankly, while the math's a bit beyond me, I'd like to see more of this kind of "How and WHY it works, or doesn't, the way it does" stuff across all the "moving parts" of the AR platform--ideally to see it distilled into a cookbook about how to select the right port size, buffer, spring etc based on your barrel length, gas length, suppressor, ammo and whether or not your BCG has 2oz of extra weight from a sidefolder.

[Shootin' Bruin]

Ray Sanchez of the suppressor company Thunder Beast has an OBC on hand and measured it to be 0.8 dB quieter on average at the shooter's ear. For rerefence, Dr. Phillip Dater of Gemtech has measured a 1-2 dB difference between a 20" barrel and 14.5" barrel at the muzzle.

Video of OBC: https://www.youtube.com/watch?v=NFyy7zOa3nU

Dater on Sound Suppression: http://www.larsondavis.com/ContentSt...d_Briefing.pdf

Ray is also playing with an accelerometer that attaches to a gun's pic rail. It will be interesting to see his results with the OBC.

[Shootin' Bruin]

Well of course if you bring the carrier velocity back up to that of the milspec carbine system, the unlock time will go back to milspec! The whole point is to reduce carrier velocity to increase unlock and extraction time.

To repeat, it's not carrier velocity that compresses the recoil spring, it's carrier energy. With that seriously heavy buffer, the velocity doesn't need to be anywhere near 6 to have milspec energy. And the milspec energy is usually quite a bit higher than necessary anyway, since most carbine systems are overgassed with 5.56. That's why carbines usually function fine with heavier-than-milspec buffers (when shooting 5.56). The milspec carbine buffer may have been designed to fit a cost constraint, and tungsten supply and fabrication would have been much more expensive than steel.

Folks with adjustable gas blocks have reported that both the LMT Enhanced BCG and Surefire OBC require less gas to operate as compared to a standard carrier. Perhaps this is due to extraction occurring at a later time when the case isn't sticking to the wall as much? If so, that could mean that the resistance from extraction plays a fairly large factor in the energy requirement needed to get the action spring to compress sufficiently enough to ensure cycling. Or could it be that the longer cam path before unlocking gives the carrier more time/distance to accelerate before meeting resistance? Perhaps both?

[WS6]

Folks with adjustable gas blocks have reported that both the LMT Enhanced BCG and Surefire OBC require less gas to operate as compared to a standard carrier. Perhaps this is due to extraction occurring at a later time when the case isn't sticking to the wall as much? If so, that could mean that the resistance from extraction plays a fairly large factor in the energy requirement needed to get the action spring to compress sufficiently enough to ensure cycling. Or could it be that the longer cam path before unlocking gives the carrier more time/distance to accelerate before meeting resistance? Perhaps both?

People with properly gassed carbines are reporting differently. One person I sold a Hodge 12.5 barrel to, could not achieve function with the LMT Enhanced carrier, and swapping to a mil-spec carrier returned proper function to the weapon. The LMT vents too much gas for a properly gassed gun.

[WS6]

Folks with adjustable gas blocks have reported that both the LMT Enhanced BCG and Surefire OBC require less gas to operate as compared to a standard carrier. Perhaps this is due to extraction occurring at a later time when the case isn't sticking to the wall as much? If so, that could mean that the resistance from extraction plays a fairly large factor in the energy requirement needed to get the action spring to compress sufficiently enough to ensure cycling. Or could it be that the longer cam path before unlocking gives the carrier more time/distance to accelerate before meeting resistance? Perhaps both?

People with properly gassed carbines are reporting differently. One person I sold a Hodge 12.5 barrel to, could not achieve function with the LMT Enhanced carrier, and swapping to a mil-spec carrier returned proper function to the weapon. The LMT vents too much gas for a properly gassed gun.

[jerrysimons]

People with properly gassed carbines are reporting differently. One person I sold a Hodge 12.5 barrel to, could not achieve function with the LMT Enhanced carrier, and swapping to a mil-spec carrier returned proper function to the weapon. The LMT vents too much gas for a properly gassed gun.

I am not sure if the Surefire OBC vents additional gas the way the LMT E carrier does. I think the E carrier vents the gas to mitigate increased carrier velocity from the altered cam path so I have always figured it mostly offset itself. I need to do more testing unsuppressed with the e carrier and conservative gassed barrels. So far E carrier does great with hodge barrels suppressed and over-gassed barrels with and without can.

[TMS951]

Thoughts on potential damage to the bolt catch from the bolt carrier traveling forward into it with momentum on the last round, as a result of the carriers over travel?