How Does The Bolt Unlock?

[Curlew]

There’s something striking to me about the pressure curves that Lysander posted above: the bullet has just barely cleared the muzzle when the gas pressure back at the carrier starts to turn on. (Compare the “cliff” of the red curve to the beginning of the lower black curve: both occur at about t=1 msec.)

That’s with an M16 and its rifle-length gas tube. For an M4 and it’s shorter, carbine-length tube, might the gas arrive at the carrier _before_ the bullet is gone? If so, could the earlier motion of the carrier cause a degradation in accuracy for the M4 vs the M16?

[lysander]

There’s something striking to me about the pressure curves that Lysander posted above: the bullet has just barely cleared the muzzle when the gas pressure back at the carrier starts to turn on. (Compare the “cliff” of the red curve to the beginning of the lower black curve: both occur at about t=1 msec.)

That’s with an M16 and its rifle-length gas tube. For an M4 and it’s shorter, carbine-length tube, might the gas arrive at the carrier _before_ the bullet is gone? If so, could the earlier motion of the carrier cause a degradation in accuracy for the M4 vs the M16?

The timing is roughly the same.



EDIT: Just a little correction/clarification: In the graphs of the piston motion and velocity, the time scale of those graphs is different from the time scales on the barrel pressures shown above and in post #7. In the velocity vs time and displacement vs time t = 0 is when the gas first enters the cavity, in the others t = 0 is primer ignition.

[Caduceus]

Geez you're all a bunch of nerds. That was probably one of the most mathematical explanations I've read here.

And frankly, I'm impressed someone knew the answer, and where to find the graphs. BZ!

[Failure2Stop]

For an M4 and it’s shorter, carbine-length tube, might the gas arrive at the carrier _before_ the bullet is gone? If so, could the earlier motion of the carrier cause a degradation in accuracy for the M4 vs the M16?

The pressurization of the gas tube and expansion chamber happens at the local speed of sound (yes, the speed of "sound" is relative and not a constant), while the projectile is traveling over 2x the speed of sound as it's passing the gas port. There is also a delay in carrier motion due to the inertia of the moving parts in conjunction with the action-spring pre-load on the operating parts. It's certainly not impossible to create a condition in an AR design where the pressures initiate carrier movement prior to projectile exit, but you'd pretty much have to either be intentionally attempting to do so or be really bad at making ARs.

One of the most accurate statements that I have heard regarding the beauty of the AR design is, "It's a true testament to the validity of the Stoner system that incompetent people can screw with the design so ineptly and it keeps working as well as it does..."

[Curlew]

There is also a delay in carrier motion due to the inertia of the moving parts

From what I can see, that’s the key. Looking at the M4 curves that Lysander posted, the gas _does_ get back to the carrier before the bullet has cleared the muzzle (vertical red line crosses the lower pressure curve), but it’s only by something like 0.1 msec. The carrier really can’t get going in so short a time — eyeballing some numbers from the graph, I figure the carrier moves only a machinist’s tenth (0.0001”) or less before the bullet is gone.

One of the most accurate statements that I have heard regarding the beauty of the AR design is, "It's a true testament to the validity of the Stoner system that incompetent people can screw with the design so ineptly and it keeps working as well as it does..."

It really is an amazingly robust and forgiving design isn’t it? With all the variations of barrel length, port location, and even caliber, it still mostly just works.

Thanks for your post.

[lysander]

The pressurization of the gas tube and expansion chamber happens at the local speed of sound (yes, the speed of "sound" is relative and not a constant), while the projectile is traveling over 2x the speed of sound as it's passing the gas port. There is also a delay in carrier motion due to the inertia of the moving parts in conjunction with the action-spring pre-load on the operating parts.

The average temperature of the gas inside the gas tube is around 2,500 K, which gives a local speed of sound in the 5,000 fps range.

[bold] At this point in the cycle, the spring's contribution is negligible. Consider the spring pre-load is, at most 10 pounds, but the gas force is around 500 pounds. According to the model, if you remove the spring entirely, the bolt does not unlock any sooner.

One of the most accurate statements that I have heard regarding the beauty of the AR design is, "It's a true testament to the validity of the Stoner system that incompetent people can screw with the design so ineptly and it keeps working as well as it does..."

Yet, Arfcom is full of people that can't seem to get their ARs to function properly . . .

[Curlew]

The timing is roughly the same.

Thanks for posting the M4 data. But the graph is indeed so similar the M16’s that I’m now questioning some things I had thought I understood....

From reading various posts here over the years, I had come to think that:

A) As compared to the M16, the M4 is harder on bolts, with failures occurring at lower round counts (broken lugs, cracks in the webs around the cam pin hole);

B) the M4 is more prone to extraction problems (hence the Crane O-ring and later the stronger copper-colored extractor spring); and

C) the cause of both A and B is a higher residual chamber pressure at the times of unlocking and extraction respectively, as a result of the shorter gas system.

But comparing the two graphs in the time interval between say t=2 and t=4 msec, they seem more alike than not. So is all of the above just bogus internet folklore? Or are A and B true, but for reasons other than C? Or is the story basically correct, but just hard to discern in the graphs?

[Failure2Stop]

The average temperature of the gas inside the gas tube is around 2,500 K, which gives a local speed of sound in the 5,000 fps range.

It is way under that temp when the pressurization occurs.

At this point in the cycle, the spring's contribution is negligible. Consider the spring pre-load is, at most 10 pounds, but the gas force is around 500 pounds. According to the model, if you remove the spring entirely, the bolt does not unlock any sooner.

The delay is in the milliseconds range, but it's enough to keep the carrier forward until well after the projectile has left the bore, even in very short barrels with low dwell. I cannot comment as to the contribution from the spring, but inertia and pre-load are contributing factors in the time to initiate movement.
I have significant and sufficient directly relevant first-hand collected data to support that statement.

[lysander]

It is way under that temp when the pressurization occurs.

Not by much.



The temperature drop is due to it expansion inside the chamber, once it vents to the atmosphere and the pressure drops to ambient it cools even more.

The delay is in the milliseconds range, but it's enough to keep the carrier forward until well after the projectile has left the bore, even in very short barrels with low dwell. I cannot comment as to the contribution from the spring, but inertia and pre-load are contributing factors in the time to initiate movement.
I have significant and sufficient directly relevant first-hand collected data to support that statement.

The gas doesn't even get to the cavity until the bullet is either gone (rifle gas system), or within 1/8 millisecond from leaving the barrel (carbine system). It takes the gas about 1-1/4 ms to move the carrier back enough to start unlocking, and another 3/4 ms to complete unlocking.

So, by the time the bolt starts to extract the empty case the bullet has been gone for about 2 ms. Total time from primer ignition to complete unlocking is about 2-3/4 to 3 ms.

If you run the model without the spring the time to complete unlocking decreases less than 5%.

EDIT:
The main point is if you have excessive bolt velocity changing the the spring to a stiffer one will not have much effect. Changing to a heavier buffer will.

[Failure2Stop]

Not by much.
https://i.imgur.com/uybSOta.png?1
The temperature drop is due to it expansion inside the chamber, once it vents to the atmosphere and the pressure drops to ambient it cools even more.

I think we are talking about different things.
I am saying that the gas inside the gas tube prior to the pressurization (before and between shots) is much lower than 2500K, thus the speed at which the pressure pulse moves down the tube to the expansion chamber is way below 5,000 f/s. This is in agreement with and supportive of:

The gas doesn't even get to the cavity until the bullet is either gone (rifle gas system), or within 1/8 millisecond from leaving the barrel (carbine system). It takes the gas about 1-1/4 ms to move the carrier back enough to start unlocking, and another 3/4 ms to complete unlocking.

So, by the time the bolt starts to extract the empty case the bullet has been gone for about 2 ms. Total time from primer ignition to complete unlocking is about 2-3/4 to 3 ms.

I don't have my timing breakdowns in front of me for the specifics, but the above generally lines up with my data.
My point was addressing the concern of carrier movement/unlock prior to projectile departure and the overall delay between the port and pressurization of the system sufficient to change anything; exactly as you lay out in the timing breakdown. It was not my intention to imply that the spring itself was the primary delay mechanism, but simply that there are several factors that contribute to the time to begin movement of the carrier, which occurs well after the projectile is out of the bore.

If you run the model without the spring the time to complete unlocking decreases less than 5%.
EDIT:
The main point is if you have excessive bolt velocity changing the the spring to a stiffer one will not have much effect. Changing to a heavier buffer will.

No disagreement, within reason of course.