AR-15 Bullet Travel vs Bolt Travel

[skeeljc]

@rammac ... thanx for a detailed reply. But I'm done, here. Experience has taught be to quit trying this discussion in the public forum. If you wish to know what I meant about the video, I'll invite you to IM me, and I'll explain privately. That way I don't bug you or anyone else further, here.

Thanx again.

That is sad. Lots of folks want to know.

 

[Dud]

I watched a Fox News video yesterday. They were looking for "the whistle blower" in the only metal whistle factory in the U.S.

It got me thinking about buying American for Christmas presents so I looked at their whistles and they have award whistles in presentation boxes with an engraved message plate.

I was looking for quotes from Coach John Wooden for the engraving. There are lots of good ones but my runner up choice seems appropriate here:

“It’s what you learn after you know it all that counts.”– John Wooden

 

[garandman]

That is sad. Lots of folks want to know.

I extend the same offer to them.

Some people just live to argue and I don't have the time . I find private discussion is the best way to avoid arguing.

 

[dellet]

The bolt can not rotate to unlock until chamber/bore pressure forcing it in to the locking lugs of the barrel extension can be overcome by the pressure pushing the bolt forward, carrier rearward + the spring pressure and buffer weight. There can be additional resistance of the case pressed against the chamber.

With enough barrel in front of the port, or a slow enough moving bullet, you might get the bolt to unlock before bullet exit, but not in general.

One of the most basic thoughts on accuracy is chamber and cartridge fit. The clearance needed to a chamber a round reliably in a semi auto, leaves the most precisely assembled round flopping around like a fish on a boat deck when compared to top end bolt action.

 

[PhilKeil]

someone with better math skills than me please check this as some math may be off a little. There are a lot of variables but using the following info i dug up

Per midway Nato xm193 ammo is rated at 3240 fps with a 55 gr bullet.
Loading the above specs in quickload a 24 in barrel barrel exit time of bullet is .991 milliseconds
standard rifle length gas tube is 15 inches
bullet passes and seals barrel port at .751 milliseconds
difference of ,24 milliseconds from after bullet passes the port to exit
Assuming the gas moves at the speed of sound, the speed of sound at sea level is 343 meters/second or .343 meters per millisecond.
For rounding the gas tube is a little less than 1/2 meter but just say 1/2 for this thought experiment.
then .343 x.5 = .1715 milliseconds before the gas begins can reach the bolt.
Now the pressure is slightly decreasing as the bullet move farther away from the port.
At the port pressure is about 18000 psi per quickload and at 24 inches it is about 7500 psi at exit

But a unknown is how much gas is needed to start the process.

I do know from my pistol AR's and wildcat AR's that are all DI and they are NOT blow back, the minimum is 10000 psi for the port location with targeted load for optimal function. Another unknown is how long for the pressure to build. but assuming moving at the speed of sound and the 10000 psi which happens to be 21.5 inches at the point it falls below the limit.
So if the bullet has .24ms to exit and it takes .1715 milliseconds that leaves about .0685 milliseconds for the bullet to complete the travel out of the tube. that equates to about 2.5 inches while this last part has to happen. please remember the pressure is decreasing more and more as it moves to exit. Now it takes another guess of .1715 milliseconds in time to build pressure just to start bolt unlock. but it takes .0685ms. to move the last 2.5 inches so i figure the bullet is ,103 milliseconds past the muzzle before unlock.

Now looking at the videos i and others posted the bullets appear to be clear of the muzzle before you see gas exiting the carrier.

While i dont doubt that the case will try to push rearward until the case grips the chamber. And there maybe some drag on the lugs as they try to rotate. but as i figure the bullet is gone before some of that happens. And there maybe some flex on on the receivers. But a previously mentioned direction is straight back. I know lock time is much longer and not going to get into that.

 

[AccelR8]

Part of me says just stay out of this but...

There are plenty of high seed camera videos on youtube of guns including AR15s firing. You can slow the motion down even more by setting the playback speed to .25 (under settings which is the little gear icon in the lower right). A couple of those videos have been posted in this thread.

It is easy to see the muzzle blast exiting before the bolt begins to move. With a little playing around, in many of these videos you can see the bullet leave the muzzle too. Here is one frame from a video someone posted here in this thread. The bullet is clearly visible as a bright streak in front of the blast, and the bolt has not begun to move and there is no gas yet being vented at the carrier.

 

[AccelR8]

Watch the firing sequence that starts about 0:27. And watch the one that begins about 0:58.

18,000 frames per second.

 

[dellet]

In an upper removed from the lower, it takes about 75 pounds of air to unlock the bolt. In an assembled rifle with a carbine buffer it's somewhere around 150. That's harder to judge because there is a point where shop air starts leaking out the gas key and it's kind of chattering back and forth.

7,000 pounds when the bullet passes the port and 2,000 at the muzzle provides plenty of inertia to cycle the action as long as the volume is there. The pressure in the gas system drops much slower than in the barrel due to the port size restricting exit. Sub-sonic loads in 300 Blackout make you rethink the gas pressures and volume needed to cycle an AR.

Does the bullet exit the barrel before the bolt moves?
How long is the dwell time?

3.75" Blackout

 

[rebel]

In an upper removed from the lower, it takes about 75 pounds of air to unlock the bolt. In an assembled rifle with a carbine buffer it's somewhere around 150. That's harder to judge because there is a point where shop air starts leaking out the gas key and it's kind of chattering back and forth.

7,000 pounds when the bullet passes the port and 2,000 at the muzzle provides plenty of inertia to cycle the action as long as the volume is there. The pressure in the gas system drops much slower than in the barrel due to the port size restricting exit. Sub-sonic loads in 300 Blackout make you rethink the gas pressures and volume needed to cycle an AR.

Does the bullet exit the barrel before the bolt moves?
How long is the dwell time?

3.75" Blackout

 

[mousegunner]

Yup. It amazes me this is controversial, and not universally known / accepted. That this discussion is even happenning.

To me this is truly apples and oranges. Make comparisons between different ARs against each other and same for bolt actions, don't jump lanes.

 

[Dud]

someone with better math skills than me please check this as some math may be off a little. There are a lot of variables but using the following info i dug up

Per midway Nato xm193 ammo is rated at 3240 fps with a 55 gr bullet.
Loading the above specs in quickload a 24 in barrel barrel exit time of bullet is .991 milliseconds
standard rifle length gas tube is 15 inches
bullet passes and seals barrel port at .751 milliseconds
difference of ,24 milliseconds from after bullet passes the port to exit
Assuming the gas moves at the speed of sound, the speed of sound at sea level is 343 meters/second or .343 meters per millisecond.
For rounding the gas tube is a little less than 1/2 meter but just say 1/2 for this thought experiment.
then .343 x.5 = .1715 milliseconds before the gas begins can reach the bolt.
Now the pressure is slightly decreasing as the bullet move farther away from the port.
At the port pressure is about 18000 psi per quickload and at 24 inches it is about 7500 psi at exit

But a unknown is how much gas is needed to start the process.

I do know from my pistol AR's and wildcat AR's that are all DI and they are NOT blow back, the minimum is 10000 psi for the port location with targeted load for optimal function. Another unknown is how long for the pressure to build. but assuming moving at the speed of sound and the 10000 psi which happens to be 21.5 inches at the point it falls below the limit.
So if the bullet has .24ms to exit and it takes .1715 milliseconds that leaves about .0685 milliseconds for the bullet to complete the travel out of the tube. that equates to about 2.5 inches while this last part has to happen. please remember the pressure is decreasing more and more as it moves to exit. Now it takes another guess of .1715 milliseconds in time to build pressure just to start bolt unlock. but it takes .0685ms. to move the last 2.5 inches so i figure the bullet is ,103 milliseconds past the muzzle before unlock.

Now looking at the videos i and others posted the bullets appear to be clear of the muzzle before you see gas exiting the carrier.

While i dont doubt that the case will try to push rearward until the case grips the chamber. And there maybe some drag on the lugs as they try to rotate. but as i figure the bullet is gone before some of that happens. And there maybe some flex on on the receivers. But a previously mentioned direction is straight back. I know lock time is much longer and not going to get into that.

The speed of sound changes with pressure. At port pressure, sound is moving just as fast as that bullet, maybe faster.

Also, as I said before, the bullet pushes the air in the barrel out at the speed of the bullet. I imagine there is a shock wave ahead of the bullet in the barrel because it is supersonic and there is nowhere else for the air to go. That shock wave might reach the bolt carrier before the bullet leaves the barrel but it will be much lower pressure than the pressure behind the bullet so it's not going to do much.

The real simple math is that we know muzzle velocity and we know carrier velocity. The muzzle velocity lets us know roughly how long the bullet is in the barrel. The carrier velocity is a peak speed with it starting at zero and accelerating until the gas key moves behind the gas tube. Assuming linear acceleration, you can do a simple integral to determine position at time. Punch in the time of bullet exit and you'll get a worst case of how far the carrier could have moved before the bullet exits. That neglects the time it takes for the pressure wave to travel through the gas tube but that would only serve to delay bolt movement. Also the bolt won't be able to move forward against the case head until the pressure in the gas tube exceeds the pressure in the barrel, delaying it even more.

The only possible complexity that would change the math is if the bullet accelerated from zero to some high speed in the barrel, passes the gas port then slows down to wait for the bolt carrier to unlock travel rearward and bottom out before the bullet again starts to accelerate and leaves the muzzle at an even higher speed.

If your bullets can do that, you probably shot JFK too.

 

[PhilKeil]

Thanks Dud. Everything i could find in short research i did talked about the pressure moving at the speed of sound. But it makes sense. Even if i recalculate it may not change much. I thought it was a good thought experiment to expand my mental horizons.

 

[dellet]

The carrier moves about 1/8” as soon as the gas system is pressurized, this is the length of travel before the cam pin is engaged at the curve in the path. It will not move any further until the bolt can rotate to unlock.

The radius of that curve determines the length time that starts extraction. This is where the “enhanced carriers” make the modifications to slow down cycling.

 

[skeeljc]

It amazes me how wrong you are and that nobody has proven it yet. I haven't found similar time information for an AR yet but if I do I will update this post. But in general, all U.S. military designed gas operated rifles will NOT open the bolt until after the bullet leaves the barrel. ALL of the action movement is after the bullet is several calibers past the muzzle. ALL of the bolt movement is directly created by momentum, not gas pressure. Gas pressure simply applies the energy that creates the inertia that starts the action's movement.

Data Item ..........................................................M1 Rifle ..............................M14 Rifle
Type of gas system .............................................gas impingement ...................gas cut-off and expansion
Location of gas port from muzzle ..........................1.5 “ .....................................8.0 “
Gas port diameter ...............................................0.0793 “ ................................0.0768 “
Barrel length ......................................................24 “ ......................................22 “
End of hammer fall ..............................................0 milliseconds ........................0 milliseconds
Bullet passes gas port .......................................1.31 milliseconds ...................1.01 milliseconds
Bullet clears muzzle ...........................................1.36 milliseconds ...................1.25 milliseconds
Initiation of operating rod recoil ...........................1.58 milliseconds ...................1.53 milliseconds
End of operating rod dwell ...................................2.89 milliseconds ..................3.57 milliseconds
Completion of bolt unlocking ...............................4.07 milliseconds ..................5.00 milliseconds
Amount of bullet travel at start of op rod recoil ......7.2 “ past the muzzle .............9.2 “ past the muzzle
Position of bullet at end of op rod dwell .............50.2 “ past the muzzle ..........76.1 “ past the muzzle
Position of bullet at completion of bolt unlocking .....88.9 “ past the muzzle ...........123.0 “ past the muzzle
Average bullet velocity ........................................2735 feet per second .............2733 feet per second

Reference: Springfield Armory Technical Note SA-TN11-1094 dated 16 Dec 1957

This is really good data. Too bad we cannot locate similar data on Eugene Stoner's design of the AR-10. No doubt it would show similar results.