Long stroke SureFire Carrier

[lysander]

In order to determine if this velocity difference does matter, we need to look at the slope of the chamber pressure at unlocking. Of course the slope of the force exerted on the chamber walls at the beginning of extraction is also relevant.

I think an examination of this latter quantity (difficult to determine in practice) would bear out the empirical result that a large increase in buffer mass does ease extraction (though perhaps not unlocking).

Also, if you compare masses of the standard carbine buffer with the A5 H4 buffer (instead of H1 vs H3), the proportional change in velocity would roughly double. The new velocity would be about 18 fps. A 10% delay in extraction and unlocking time could well be relevant.

However, if that 20 fps was the minimum carrier velocity, you are going to have to bump up the acceleration to regain the velocity.

In any case, the time difference between a rifle and carbine is 1/2 a millisecond, that's 30% of the total time to unlock for a rifle gas system. You can't make up that much time just by changing the weight. If you add enough weight to make that big a time delay, the velocity drops to the point where you need more gas, which increases the acceleration of the carrier, which will, in turn, move the time back down.

This is why piston systems like the HK or the AR-180 use a gap between the piston and carrier, or longer cam tracks to get the required dwell. Compare the cam track for an AR-15 and the AR-180, the free-run (the straight portion of the cam track before the actual helical cam) is 0.07" for an AR-15, and .105" for the AR-180. The AR-180 has 50% more free run, in addition to the 0.020" to 0.050" gap between the piston rod and carrier to delay unlocking.

[lysander]

Oh, and one more thing about "dwell".

In the text books on automatic gun design, dwell is defined as the time between primer ignition and unlocking. How the AR community morphed the term to mean the time between the bullet uncovering the gas port and bullet exit, I don't know. In any case, the AR community's definition is not only wrong, it is a useless factor.

Let's look at this figure of the M4 gas system again (14.5 inch barrel with carbine length gas tube):

(Click on image to enlarge)

The heavy red line is the pressure on the base of the bullet as it travels down the barrel, and not unexpectedly, on bullet exit it drops to zero rather quickly. The heavy black line is the measured pressure in the chamber, It has a small kink at bullet exit but largely drops off at the same rate as before. The yellow line is the pressure at the entrance of the gas tube, it has a kink at bullet exit, as well, but it is smaller. The blue line is the pressure at the exit of the gas tube, if there is a kink, it is now too small to show on these graphs, and the pressure rises and falls in a smooth curve. The lowest black line is the pressure at the entrance of the carrier cavity, again there is no discontinuity at bullet exit, and the pressure in the carrier does not even reach its peak until 1/4 millisecond after the bullet has gone.

The addition of an extra 1.5 inches on the barrel are not going to make any difference back in the bolt carrier.

[StainlessSlide]

However, if that 20 fps was the minimum carrier velocity, you are going to have to bump up the acceleration to regain the velocity.

There isn't a minimum carrier velocity, only a minimum carrier energy, and we were keeping energy constant by increasing mass and decreasing velocity appropriately.

As far as the comparison between carbine and rifle go, we don't know which portion of that 1/2 millisecond leads to the decreased extraction force.

You wrote that unlocking begins at 1.75 milliseconds, which looks like about 1 ms after the expansion chamber begins to be pressurized. A 10% reduction in bcg velocity profile gives a gain of .1 ms there, admittedly only 1/5 of the rifle-carbine difference.

But of course extraction by the bcg does not occur until the cam reaches the end of the track. An average bcg velocity of 10 ft/sec during the period between unlocking and extraction applied to a distance of .3 in between unlocking and extraction, gives a time difference of of 2.5 ms between unlocking and extraction. This gives a time of 3.5 ms between expansion chamber pressurization and extraction. A 10% difference in the velocity profile here yields a .35 ms lengthening of extraction time, 70% of the way from carbine to rifle time.

Here I am assuming that carbine and rifle bcg velocities are equal, so that a .5 ms difference in unlocking time implies a .5 ms difference in extraction time. This may not be right.

I would also like to point out that the time gap between the bullet passing the port and the start of bcg pressurization looks like about .1 ms. For the distance involved, this represents a velocity of about Mach 5. A supersonic shock is running down the gas tube, with supersonic flow behind it.

[lysander]

There isn't a minimum carrier velocity, only a minimum carrier energy, and we were keeping energy constant by increasing mass and decreasing velocity appropriately. [1]

As far as the comparison between carbine and rifle go, we don't know which portion of that 1/2 millisecond leads to the decreased extraction force.

You wrote that unlocking begins at 1.75 milliseconds, which looks like about 1 ms after the expansion chamber begins to be pressurized. A 10% reduction in bcg velocity profile gives a gain of .1 ms there, admittedly only 1/5 of the rifle-carbine difference.[2]

But of course extraction by the bcg does not occur until the cam reaches the end of the track. An average bcg velocity of 10 ft/sec during the period between unlocking and extraction applied to a distance of .3 in between unlocking and extraction, gives a time difference of of 2.5 ms between unlocking and extraction. [3] This gives a time of 3.5 ms between expansion chamber pressurization and extraction. A 10% difference in the velocity profile here yields a .35 ms lengthening of extraction time, 70% of the way from carbine to rifle time.

Here I am assuming that carbine and rifle bcg velocities are equal, so that a .5 ms difference in unlocking time implies a .5 ms difference in extraction time. This may not be right.[4]

I would also like to point out that the time gap between the bullet passing the port and the start of bcg pressurization looks like about .1 ms. For the distance involved, this represents a velocity of about Mach 5. A supersonic shock is running down the gas tube, with supersonic flow behind it.[5]

1) There are two things required for proper operation, you have to have enough momentum and energy, and they are related, but different. Since the mass is fixed, the is a minimum velocity to achieve minimum momentum.

2) The average velocity is not a very good assumption. That would assume the acceleration is constant, it isn't, the initial acceleration is significantly higher in the beginning and drops as the volume increases.

3) Extraction is not what we are worried about here. The timing at the initiation of unlocking is the important thing. If the chamber pressure is forcing the lugs against the barrel extension there is a torsional load on the bolt, if the two forces are balanced there is no twisting load on the bolt. So, the time interval we are interested in the first 0.070 inch of carrier travel. When the bolt carrier moves rearward 0.070" the cam pin enters the helical portion of the cam track and begins to twist the bolt. That point is approximately when the cavity pressure just reached its maximum value. In order to reduce the torsional load on the bolt requires the time to this maximum has to be increased 0.5 ms. You cannot slow the carrier down that much and still get it to cycle.

4) No, as stated the acceleration is not constant. It starts very high and about half way through the cavity pressurization, it drops considerably. By the time extraction starts, the acceleration is just starting to go negative, as the carrier cavity pressure is venting, the carrier has picked up the mass of the bolt and case and must transfer momentum to them, and there is the drag force from the case/chamber.

5) First of, gas flow through a tube cannot move at supersonic speed, it's maximum speed is restricted to Mach 1, period.

The point where the yellow line (port-barrel interface) rises from zero is when gas first enters the gas tube. The point where the black line (cavity/key interface) rises from zero is when the gas starts to exit the gas tube. The time interval between these two points is about 0.2 ms.

The gas tube is 7.5 inches long.

The speed the gas is traveling is:

v = d/t

d = distance, 7.5 inches
t = time, 0.2 ms 0.0002 seconds

v = 7.5/.0002 = 37,500 in/sec = 3125 fps.

Speed of sound is dependent of the temperature of the gas. The gas temperature in the tube is around 2500 degrees C, therefore, the velocity for Mach 1 at this temperature is around 3463 fps. If you reverse the calculations the time require for gas at 3463 fps to go 7.5 inches is 0.18 ms. Almost exactly what the graph shows.



EDIT: AN ASIDE: A few more things more about 16 inch carbine systems:

The muzzle velocity of M855 from a 14.5" barrel is officially listed as 2970 fps. From a 16" it will be faster as the bullet is still accelerating. For simplicity sake, let's assume the bullet averages 2900 fps for the last 1.5 inches of barrel. How long will it take for a bullet at that speed to travel 1.5 inches?

d =v x t

v = velocity, 2900 fps, or 34,800 inches/second
d = distance, 1.5 inches.

solve for t.

d/v = t

1.5 / 34800 = 0.04 milliseconds, 40 millionths of a second.

That small a time interval is not going to matter in how long the 'gas tube is pressurized'...

[TMS951]

I received and installed mine.

First impression was it’s quieter to the ear than with the LMT carrier. This was just my unscientific impression, I wasn’t looking for this in particular, just the first thing that popped to mind when I started shouting it.

Nothing long term to note yet. I’ll chime back in after 1000 rounds.

[Walker_Texasranger]

I wonder how this would pair with an A5 buffer? Not sure if it’d result in ultimate smoothness or if it’d lead to reliability issues in something like a 16” mid length.

[StainlessSlide]

1) There are two things required for proper operation, you have to have enough momentum and energy, and they are related, but different. Since the mass is fixed, the is a minimum velocity to achieve minimum momentum.

2) The average velocity is not a very good assumption. That would assume the acceleration is constant, it isn't, the initial acceleration is significantly higher in the beginning and drops as the volume increases.

3) Extraction is not what we are worried about here. The timing at the initiation of unlocking is the important thing. If the chamber pressure is forcing the lugs against the barrel extension there is a torsional load on the bolt, if the two forces are balanced there is no twisting load on the bolt. So, the time interval we are interested in the first 0.070 inch of carrier travel. When the bolt carrier moves rearward 0.070" the cam pin enters the helical portion of the cam track and begins to twist the bolt. That point is approximately when the cavity pressure just reached its maximum value. In order to reduce the torsional load on the bolt requires the time to this maximum has to be increased 0.5 ms. You cannot slow the carrier down that much and still get it to cycle.

4) No, as stated the acceleration is not constant. It starts very high and about half way through the cavity pressurization, it drops considerably. By the time extraction starts, the acceleration is just starting to go negative, as the carrier cavity pressure is venting, the carrier has picked up the mass of the bolt and case and must transfer momentum to them, and there is the drag force from the case/chamber.

5) First of, gas flow through a tube cannot move at supersonic speed, it's maximum speed is restricted to Mach 1, period.

The point where the yellow line (port-barrel interface) rises from zero is when gas first enters the gas tube. The point where the black line (cavity/key interface) rises from zero is when the gas starts to exit the gas tube. The time interval between these two points is about 0.2 ms.

The gas tube is 7.5 inches long.

The speed the gas is traveling is:

v = d/t

d = distance, 7.5 inches
t = time, 0.2 ms 0.0002 seconds

v = 7.5/.0002 = 37,500 in/sec = 3125 fps.

Speed of sound is dependent of the temperature of the gas. The gas temperature in the tube is around 2500 degrees C, therefore, the velocity for Mach 1 at this temperature is around 3463 fps. If you reverse the calculations the time require for gas at 3463 fps to go 7.5 inches is 0.18 ms. Almost exactly what the graph shows.



EDIT: AN ASIDE: A few more things more about 16 inch carbine systems:

The muzzle velocity of M855 from a 14.5" barrel is officially listed as 2970 fps. From a 16" it will be faster as the bullet is still accelerating. For simplicity sake, let's assume the bullet averages 2900 fps for the last 1.5 inches of barrel. How long will it take for a bullet at that speed to travel 1.5 inches?

d =v x t

v = velocity, 2900 fps, or 34,800 inches/second
d = distance, 1.5 inches.

solve for t.

d/v = t

1.5 / 34800 = 0.04 milliseconds, 40 millionths of a second.

That small a time interval is not going to matter in how long the 'gas tube is pressurized'...

This is quite a fruitful discussion. Excellent. I wish we had a better medium of communication.

Answering your points one by one,

1) There is no reason that there should be a minimum momentum. Energy is required to compress the recoil spring and to go into the various dissipations. After the expansion is completed, this will come from the kinetic energy of the bcg. If the kinetic energy is there, the bcg will complete the cycle.

Of course in all of our arguments we are making simplifying assumptions, so all results will be approximations.

By the way, there's no need to remind me that energy and momentum are different, but related. Of course in simple Newtonian mechanics of free particles the relation is E = p^2/2m. The generalized momentum in the Hamiltonian or Lagrangian formulation is more complex. In systems with potentials and dissipations, other types of energy than kinetic are important.

2) You are correct that the velocity function will be nonlinear. I did not think that the slow rise and slower fall in pressure (and thus acceleration) would change the result significantly. I was not looking for an exact result, but one good to the accuracy of our discussion. This part of my argument did not bear on unlocking time (I used your number for this), only on extraction time.

3) I see that you were only interested (in your discussion) in zeroing the torsion load on the bolt. I was analyzing the reliability increase of a heavier buffer in general.

The .1 ms delay that I calculated will certainly not zero the load on the bolt lugs at unlocking, but could still be significant in extending bolt life and reducing the variance of the frictional dissipation at unlocking. Both of these effects are probably heavily nonlinear in this quantity, and the first .1 ms could makes a lot of difference. Hard to say.

There is empirical evidence that a heavier buffer in a carbine increases reliability in general (which includes extraction force). It seems proven that the rifle has longer mean bolt life than the carbine. But does the heavier buffer extend bolt life? There are so many confounding variables.

4) The nonlinearity of the velocity function is not relevant here. I was asserting that if the velocity function of the rifle and carbine bcgs were equal, then the time difference between unlocking and extraction would be equal, and thus a .5 ms delay in unlocking (as you write that the rifle has) would lead to a .5 ms delay in extraction.

Of course the bcg will decelerate during extraction. The venting is not relevant during extraction, since the cam is at the end of the track, and expansion chamber pressure can no longer accelerate the bcg, no matter what its level. As an another aside, it's hard to imagine that the buffer and bcg will remain in contact at this point. This would only happen if the extraction force was less than the recoil spring force.

5) The designers of supersonic wind tunnels and shock tubes would be quite surprised to learn that supersonic flow in a tube is not possible, period. What is true is that the flow cannot accelerate through Mach 1 in a straight sided tube.

The gas we are dealing with flows through a constriction at the gas port which may not have sharp corners after a while. Supersonic flows increase in velocity as a tube widens, contrary to the subsonic Bernoulli effect. This is a complicated flow.

You may be right, the flow may not be supersonic, but the shock moving up the gas tube certainly is. Shock velocities are compared to the speed of sound in the medium into which the shock is moving.

I read your graph as having a travel time down the tube of closer to .1 ms than to .2 ms.

You didn't take into account the molecular weight of the gas flowing from the gas port into the gas tube (or the ratio of specific heats) in your sound speed calculation. The gas is definitely not air. This makes the calculation very difficult, since surely the molecular weight depends on the completeness of the combustion. And this may be a dusty flow.

None of this is a true dynamic analysis, with everything bouncing around. But maybe it is good enough.

This is turning into quite a nerd fight. I've done a lot of thinking, and learned a lot.

[lysander]

First of, gas flow through a tube cannot move at supersonic speed, it's maximum speed is restricted to Mach 1, period.

Typo - not "a tube" but "the tube" the flow is choked in the gas tube. Just because it expands and accelerates in one portion (just after the barrel port, and as it enters the carrier cavity) does not mean cannot get restricted elsewhere.

And, the piston vents about 3/4 of the piston stroke, this is before the bolt starts to move. But even with the vent ports exposed the pressure does not drop to zero instantly, in fact, it takes about 4 ms to completely decay to zero, but it does reduce the carrier acceleration. It is designed to do that to reduce the impact load on the cam pin.

The upshot of my point is:

A heaver reciprocating mass does not, and cannot, change the time at which the bolt unlocks to any large degree. That has has to be handled by distance to the gas port, cam track design, and other mechanical delays.

There is empirical evidence that a heavier buffer in a carbine increases reliability in general (which includes extraction force).

This is because it usually slows the bolt velocity towards the minimum cyclic rate. Doing this helps eliminate your feed related malfunctions, also, a heavier mass moving forward has more momentum for stripping rounds out of the magazine with less velocity loss.

And, the extraction force is defined as the force required to pull the fired case from the chamber. This is governed by the case/chamber friction and residual chamber pressure, and cannot be changed by adding or subtracting weight from the carrier. I figure you are actually saying that heavier buffers increase to reliability of extraction, that would make sense as the more momentum you have, the better you can pick up additional mass without large velocity loss.

If you really feel like modeling the system: (believe it or not you can do it in Excel)

The Gas Flow in Gas-Operated Weapons

Sensitivity Study of Rifle Gas Systems

Experimental and Theoretical Study of Parameters Affecting Operation of Basic Gas System

[TMS951]

So can you guys calculate how much later these carriers cause the bolt to unlock?

[BallisticHarmony]

When using this in a Knight's Armament MOD 2 upper and E3 bolt, I've read on M4C before that KAC only recommends running their own buffers, since that's what they've tuned their gas systems to.

Would that also be the case with this BC? So many different choices now on how to make your gun run as optimally as possible, it's a little overwhelming. Shouldn't reliability always be a higher priority than recoil reduction, outside of the competition world? So maybe the smoothest shooter isn't always the best option?