Long stroke SureFire Carrier

[StainlessSlide]

A heaver reciprocating mass does not, and cannot, change the time at which the bolt unlocks to any large degree. (1) That has has to be handled by distance to the gas port, cam track design, and other mechanical delays.

This is because it usually slows the bolt velocity towards the minimum cyclic rate. (2) 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.(3)

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. (4) 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.

[/URL]

Those papers are a treasure trove! Thank you for posting them.

(1) An arbitrarily large buffer weight could change the unlock time to any desired value. A large mass can have low velocity and still have enough energy to do the work. The low velocity delays the unlock time.

I agree that the available buffer weights can only delay unlock time by about .1 ms. Whether this is enough to have an impact on bolt life or cycling reliability is best determined empirically.

(2) There is no minimum cyclic rate (in a theoretical sense). Again, a more massive bcg can operate at lower velocities, and still have enough energy to complete the cycle. The lower velocities will lower the cyclic rate.

Compressing the recoil spring takes a certain amount of integral Force(x) dx = Work. Work has dimensions of energy. It's the energy necessary to exert a force through a distance. Velocity does not do work.
A mass could have a velocity of 1000 ft/sec, and lack the energy to lift a mosquito 1 inch. Another mass could have a velocity of .000001 ft/sec and still have the energy to lift the Empire State Building 500 ft.

Momentum does not do work. A mass with momentum of 1000 slug ft/sec could lack the energy to lift the mosquito 1 inch, if the mass was large enough.

Bcg energy is the relevant quantity, and enough energy can be had at lower velocities if the mass is increased.

I think the problem may be that the papers you reference do talk about the momentum of the bolt carrier. They use the momentum as a measure of energy. This is valid via E = p^2/2m, as long as the mass is constant. They are talking about a constant mass bcg, the M16 mass. We are talking about varying the mass.

If mass is constant, velocity is also a measure of energy (not a linear one). But again we are talking about varying the mass.

(3) You can probably anticipate what I'm going to say here. It takes a certain amount of energy (not momentum) to strip the round.
If I compress the recoil spring by an amount x past the relaxed length, the spring stores energy V = (1/2) k x^2, where k is the spring constant. If I then put a big or small mass ahead of the spring and let go, the spring will give this energy V to the big or the small mass. This will lead to a difference in velocity between the two, but not a difference in energy.

If the recoil spring has been fully compressed, a larger or smaller mass bcg will have the same energy at each point in its forward travel (but not the same velocity).

(4) Again a more massive bcg can operate at a lower velocity (10% lower according to my analysis above), delaying extraction time (by .35 ms or so, according to my calculation above) and thus giving more time for the chamber pressure to drop and the brass to cool and contract. This will reduce extraction force.

It looks like you are right about the flow in the gas tube being subsonic, except for a short length at the beginning.

Some of my statements above involve approximations to keep things simple, but I think the general conclusions are valid. Forgive me if I have belabored my points.

[lysander]

Newton's Second Law

Conservation of momentum.

m1 x v0 = (m1 +m2) x v1

with:

m1 - bolt/carrier/buffer mass
m2 - cartridge mass
v0 - initial velocity
v1 - velocity after picking up the cartridge

The larger m1 is relative to m2 the smaller the difference between v0 and v1. The smaller the difference between v0 and v1, the smaller the energy lost to picking up the round from the magazine.

The momentum of a body is directly proportional to the force applied.

F = d(mv)/dt => d(m)/dt x d(v)/dt

-or-

Force equals the time rate of change of mass times the time rate of change of velocity.

The force required to strip a round from the magazine is essentially fixed*, so the larger the initial mass the smaller deceleration. The smaller the deceleration the smaller the velocity drop, the smaller amount of energy lost.

Therefore, any increase in bolt carrier or buffer mass (momentum) will increase performance.

And, there is a minimum practical cyclic rate, otherwise we would not have threads about weak extraction, poor feeding, and reports from people about ejection at 5:00...

We are talking about varying the mass.

???????

The mass of the bolt/carrier/buffer does not vary, it is fixed during the cycle. If you change the buffer, you redo all the calculations, and get new momentums.

____________________
* it does depend on the position of the round in the magazine, last round of 30 or first round or 30...but for the purpose of this discussion we will assume the round in question is always the first one of a 30 round magazine.

[lysander]

Okay, here is a piece of my model of an AR-180 showing just how little changing mass has on the start of unlocking. All parameters are unchanged except the mass of the bolt carrier was increased by 2 ounces (roughly the equivalent of going from and H1 to an H3 buffer).

The time to start unlocking is increased by .05 milliseconds, 5/100,000 second.
The time to the completion of unlocking is increased by .13 milliseconds.

Remember, the time delay required to get a carbine length gas system to match the time for a rifle length gas system was 0.5 ms, half a millisecond



Tripling the carrier mass, going from a little under a pound to three (3) pounds gets you the full half millisecond, but drops the carrier velocity from 20 fps to 8 fps. Since the energy is related by the square of the velocity, you have lost a lot of energy.

Like I said, you can't gain unlocking delay by increasing mass, at least with any realistic weight increase.

[Clint]

Since you have a model, can you plug in the parameter for lengthening or shortening the gas port/piston location by 1 or 2 inches and replot?

Okay, here is a piece of my model of an AR-180 showing just how little changing mass has on the start of unlocking. All parameters are unchanged except the mass of the bolt carrier was increased by 2 ounces (roughly the equivalent of going from and H1 to an H3 buffer).

[lysander]

My model is an AR-180 with minor changes to suit what I was designing, so no, I can't.

But, the principle is the same as an AR. A carrier mass, with a similar pressure (therefore force) profile, and fixed unlocking geometry.

However, if you go back a page or so I posted the profiles of an M16 and an M4, those give you a very good idea. You're talking about half the distance between rifle and carbine.

[StainlessSlide]

F = d(mv)/dt => d(m)/dt x d(v)/dt

-or-

Force equals the time rate of change of mass times the time rate of change of velocity.

No, F = d(mv)/dt => (d(m)/dt) x v + (d(v)/dt) x m ******** Remember to use the product rule

???????

The mass of the bolt/carrier/buffer does not vary, it is fixed during the cycle. If you change the buffer, you redo all the calculations, and get new momentums.

Wow, that's a lot of question marks! Changing the cycle by varying the effective bcg mass by varying the buffer mass is what this whole conversation has been about. And yes, the fact that changing the buffer results in different momenta (but the same energy, if we do it right), is the whole point.


Sure, momentum is conserved and all of Newton's Laws hold. But momentum is not the fundamental quantity here, nor is velocity.

I could design a system similar to an AR that operated at an initial bcg velocity of 1 fps or 100 fps, at a momentum of 14 slug ft/sec or .12 slug ft/sec. It would still extract the brass frrom a milspec AR barrel, cock the hammer, fully compress the milspec recoil spring, feed the next round, compress the ejector spring, and pop the extractor over the rim. I would definitely have to change the bcg mass (and also I should increase the rifle mass at one of these extremes, since we need the assumption that the rifle mass is much greater than the bcg mass).

I could not design a system that did all this if the initial bcg energy was less than the sum of the fixed and well defined energies necessary to compress these springs and free the first round from the mag lips, and the less well defined energies which will cover the dissipations inherent in the system. Obviously the above is a thought experiment to make a point, rather than a practical proposal. But it could be done (energetically anyway).

If you are analyzing the performance improvement of changing bcg mass, the right way would be

1. Make an energy budget of these quantities
2. Increase your value of the bcg mass (thereby reducing the operating velocities and delaying unlocking and extraction)
3. Increase your port size to make up any minor energy deficit, caused by the fact that the gas system is delivering a bcg energy somewhere between constant-impulse and constant-energy.

And, there is a minimum practical cyclic rate, otherwise we would not have threads about weak extraction, poor feeding, and reports from people about ejection at 5:00...

There is no reason that a low cyclic rate should cause weak extraction or poor feeding, as long as the bcg energy is there. This would be the case if the low cyclic rate were caused by increased bcg mass. Of course some small energy correction would be necessary in this case, see #3 above.

Think of a simple harmonic oscillator. The period is T = 2 Pi root(m/k). It does not depend on the energy (or the initial velocity at the neutral point), but does depend on the mass. This is not a perfect analogy for the AR cycle, since we are not starting from the neutral point (since the recoil spring is preloaded), but it gets my idea across.

[StainlessSlide]

Okay, here is a piece of my model of an AR-180 showing just how little changing mass has on the start of unlocking. All parameters are unchanged except the mass of the bolt carrier was increased by 2 ounces (roughly the equivalent of going from and H1 to an H3 buffer).

The time to start unlocking is increased by .05 milliseconds, 5/100,000 second.
The time to the completion of unlocking is increased by .13 milliseconds.

Remember, the time delay required to get a carbine length gas system to match the time for a rifle length gas system was 0.5 ms, half a millisecond

Tripling the carrier mass, going from a little under a pound to three (3) pounds gets you the full half millisecond, but drops the carrier velocity from 20 fps to 8 fps. Since the energy is related by the square of the velocity, you have lost a lot of energy.

Like I said, you can't gain unlocking delay by increasing mass, at least with any realistic weight increase.

I would have to see your model, but my calculations above were similar, yielding a .1ms unlocking delay from an increase of 3oz. in an AR system. Again, I acknowledged above that a practical increase in buffer mass wasn't going to get us to a .5ms unlocking delay. The question was whether a .1 ms delay was beneficial.

Your tripling of the carrier mass is obviously intended as another thought experiment, so I'll continue in that vein.

Sure, your energy has decreased by a factor of about 2.08. Open the gas port to boost the initial bcg velocity to 11.5 ft/sec or so, and now your energy is the same, at lower velocity, so you still have a nice unlocking delay.

Increase the carrier mass sixfold, open the port to keep energy constant, and you'll probably get to .5ms delay, in this thought experiment. This illustrates the point I was making about operating at lower velocity but equal energy (but not at this extreme).

Returning to the practical world, most ARs have energy to spare when using 5.56, so a minor energy drop caused by going to a heavier buffer will probably not be a problem.

Can you get a value for the extraction delay? It would be interesting to see what your model says about that.

[lysander]

The maximum torsion on the bolt is directly related to the residual chamber pressure at the start of unlocking, not at the end.

How much does that decay over the .05 ms?

I haven't bothered to run the model longer than the start of extraction. I just tailored the design to achieve a bolt/carrier momentum that matches the M16 rifle system, and the resulting prototype worked just fine. Maybe some day I'll dust it off and play with it some more . . . .

[StainlessSlide]

Thank you Lysander for a stimulating exchange. Quantification of the unlocking delay is not something I had seen before.

[Shootin' Bruin]

Here is a video that discusses the relationship among reciprocating mass, cycle distance, cycle time, and impulse. A practical demonstation is shown at the 11 minute mark: https://www.youtube.com/watch?v=4W-e...L&index=2&t=0s

Tying in StainlessSlide and lysander's discussion to the original topic of this thread, I would like to add that the carrier's increase in free travel before the bolt begins to unlock is what allows both the Surefire OBC and LMT's Enhanced Bolt Carrier to delay unlocking even if reciprocating mass were unaltered. Of course, the revised cam paths of these carriers would also magnify any delay in unlocking caused by an increase in mass, perhaps to a point significant enough to positively affect extraction reliability.

Since Surefire's system has both the revised cam path and increased mass, it would be nice to see them prove their claims by setting up an AR to be internationally overgassed to the point that it will no longer extract reliably with a standard BCG (say a 10.3" barrel with a .09" gas port) and then swapping their carrier in.

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?

Those papers are a treasure trove! Thank you for posting them.

(1) An arbitrarily large buffer weight could change the unlock time to any desired value. A large mass can have low velocity and still have enough energy to do the work. The low velocity delays the unlock time.

I agree that the available buffer weights can only delay unlock time by about .1 ms. Whether this is enough to have an impact on bolt life or cycling reliability is best determined empirically.

(2) There is no minimum cyclic rate (in a theoretical sense). Again, a more massive bcg can operate at lower velocities, and still have enough energy to complete the cycle. The lower velocities will lower the cyclic rate.

Compressing the recoil spring takes a certain amount of integral Force(x) dx = Work. Work has dimensions of energy. It's the energy necessary to exert a force through a distance. Velocity does not do work.
A mass could have a velocity of 1000 ft/sec, and lack the energy to lift a mosquito 1 inch. Another mass could have a velocity of .000001 ft/sec and still have the energy to lift the Empire State Building 500 ft.

Momentum does not do work. A mass with momentum of 1000 slug ft/sec could lack the energy to lift the mosquito 1 inch, if the mass was large enough.

Bcg energy is the relevant quantity, and enough energy can be had at lower velocities if the mass is increased.

I think the problem may be that the papers you reference do talk about the momentum of the bolt carrier. They use the momentum as a measure of energy. This is valid via E = p^2/2m, as long as the mass is constant. They are talking about a constant mass bcg, the M16 mass. We are talking about varying the mass.

If mass is constant, velocity is also a measure of energy (not a linear one). But again we are talking about varying the mass.

(3) You can probably anticipate what I'm going to say here. It takes a certain amount of energy (not momentum) to strip the round.
If I compress the recoil spring by an amount x past the relaxed length, the spring stores energy V = (1/2) k x^2, where k is the spring constant. If I then put a big or small mass ahead of the spring and let go, the spring will give this energy V to the big or the small mass. This will lead to a difference in velocity between the two, but not a difference in energy.

If the recoil spring has been fully compressed, a larger or smaller mass bcg will have the same energy at each point in its forward travel (but not the same velocity).

(4) Again a more massive bcg can operate at a lower velocity (10% lower according to my analysis above), delaying extraction time (by .35 ms or so, according to my calculation above) and thus giving more time for the chamber pressure to drop and the brass to cool and contract. This will reduce extraction force.

It looks like you are right about the flow in the gas tube being subsonic, except for a short length at the beginning.

Some of my statements above involve approximations to keep things simple, but I think the general conclusions are valid. Forgive me if I have belabored my points.