How Does The Bolt Unlock?

[prepare]

Right at 4000 F.

Thanks.

Now wonder the gas tube turns red.

[lysander]

Thanks.

Now wonder the gas tube turns red.

Gas at that temperature in only in the tube for a fraction of a millisecond, it takes time for heat to transfer from the gas to the walls of the gas tube. Further, the heat is transferred through the stainless steel much faster than the heat is transferred from the gas to the tube, making the tube a sizable heat sink. So, it takes a few hundred rounds to get the tube really hot. And, as soon as there is a lull in the firing, convection and radiation will begin to dump the heat.

[prepare]

In some of the endurance/destructive experiments I've seen gas tubes last for approximately 700-900 rounds of continuous firing before rupturing.

[Failure2Stop]

Yes, the graphs were generated by FLUENT. The data presented is from basically four reports:

1) "The Gas Flow in Gas Operated Weapons," BRL, 1970.
2) "Sensitivity Study of Rifle gas Systems", BRL, 1971
3) "Comparison of a Theoretical and Experimental Study of the Gas system in the M16A1 Rifle," BRL, 1971.
4) "Numerical Modeling of the Gas Flow in the M4 and M16 Rifle System.", ARDEC, 2009.

From actual testing, combined use of sensors and high speed video:

Carbine Gas 10.5 M855
Time
0.0000____Begin combustion
0.0002____Max pressure from combustion
0.0006____Max pressure gas port
0.000833__Start of bolt carrier motion
0.001333__Begin cam pin rotation
0.002667__Extraction
0.021167__Ejection (perpendicular to bolt)
0.021333__Carrier Rearward Travel Stops

KAC Intermediate Gas 16in M855
Time
0.0000____Begin combustion
0.0002____Max pressure from combustion
0.0008____Max pressure gas port
0.001167__Start of bolt carrier motion
0.001667__Begin cam pin rotation
0.003167__Extraction
0.023667__Ejection (perpendicular to bolt)
0.031667__Carrier Rearward Travel Stops

[prepare]

After max pressure gas port the numbers really begin to change.

[lysander]

From actual testing, combined use of sensors and high speed video:

Carbine Gas 10.5 M855
Time
0.0000____Begin combustion
0.0002____Max pressure from combustion
0.0006____Max pressure gas port
0.000833__Start of bolt carrier motion
0.001333__Begin cam pin rotation
0.002667__Extraction
0.021167__Ejection (perpendicular to bolt)
0.021333__Carrier Rearward Travel Stops

KAC Intermediate Gas 16in M855
Time
0.0000____Begin combustion
0.0002____Max pressure from combustion
0.0008____Max pressure gas port
0.001167__Start of bolt carrier motion
0.001667__Begin cam pin rotation
0.003167__Extraction
0.023667__Ejection (perpendicular to bolt)
0.031667__Carrier Rearward Travel Stops

The times for the carbine gas system are very similar to the results found in the Army reports and what is calculated from the models they used. Below are blow-ups from the two previous graphs in posts #7 and #12, The jaggy lines are the transducer trace, the smooth lines are the model results. For the carbine gas system the peak chamber pressure and peak port pressures are timed almost exactly what you posted for a similar gas system. They give 0.00025 and 0.00075 you give 0.0002 and 0.0006. (but, you gave a 10.5 in bbl so a bigger gas port?)



Going back a little further to post #5 we see extraction starts at the end of piston travel, at t' = .0025 sec, but t' = 0 when the gas first enters the cavity so t' = t + .001 sec, with t = 0 at ignition. But, this is for a rifle gas system. If we look at the graph in post #12 we see that the gas first enters the cavity in the carbine gas system at t = 0.0006, so my model shows extraction starting at 0.0031 seconds, compared to your 0.0026.

Since the barrel lengths, buffer weights, gas ports, and exact chamber pressure profiles might be different, and the fact that pulling accurate times and pressures off a small graph are not the pinnacle of accuracy, that difference between your hard numbers and my model of half a millisecond is well within the accuracy tolerance. With pressure-time curves pulled out of something like QuickLoad you can build a fairly decent model of any gas operated weapon, because when you boil it down to the basics all of them look like this:

[Bears4deblife89]

What brings this up?

[Disciple]

This. This is one of the coolest threads on the ‘net.

My thanks as well.

[lysander]

From the model:





Carbine length gas system 14.5 inch barrel M855

Time ------ Event
0.00000 -- Ignition
0.00070 -- Start of bolt carrier motion
0.00160 -- Bolt begins to rotate
0.00283 -- unlocking complete
0.00310 -- Extraction starts

If you remove the spring:

Time ------ Event
0.00000 -- Ignition
0.00065 -- Start of bolt carrier motion
0.00155 -- Bolt begins to rotate
0.00280 -- unlocking complete
0.00297 -- Extraction starts

[Failure2Stop]

From the model:

Carbine length gas system 14.5 inch barrel M855
Time ------ Event
0.00000 -- Ignition
0.00070 -- Start of bolt carrier motion
0.00160 -- Bolt begins to rotate
0.00283 -- unlocking complete
0.00310 -- Extraction starts

If you remove the spring:

Time ------ Event
0.00000 -- Ignition
0.00065 -- Start of bolt carrier motion
0.00155 -- Bolt begins to rotate
0.00280 -- unlocking complete
0.00297 -- Extraction starts

Good visualization of how fine of an adjustment to the overall system operation that spring swapping does.