Piston force of the bolt–carrier system

[Disciple]

What is the peak force produced by the bolt–carrier piston system? Searching for an answer I am finding conflicting results.

The force pushing the bolt forward is the same as the force pushing the BCG rearward.

So exactly how little force do you think it takes to accelerate the mass of the bolt carrier?

Do the math and see for yourself.

It's a LOT.

Here are a few round starting values to calculate it both ways:

The BCG with a mass of 0.5 kg accelerates to a speed of 4 m/s in 200 micro seconds. What force is required?

The piston has an effective diameter of .43" and an average pressure of 15000 psi. What force is exerted?

This yields 2178 lbf.

The pressures at the gas ports are: 13.5K for the
rifle and 26K for the carbine -- or twice as much.

The dwell time (the time that the gas system is
charged with high pressure) is determined by the
amount of barrel after the gas port. These are nearly
identical between the rifle and the carbine.

Pressure from the port is regulated only by the size
of the gas port and the diameter of the barrel.

These two factors determine the internal bolt
pressure, the maximum pressure that is obtained in the
bolt carrier/piston combination -- for the rifle this
pressure is about 1000psi and for the carbine it is
over 1500psi, half again as much.

Are 1000psi and 1500psi errors? This is an order of magnitude less than the value Clint gave.

The piston force is on the order of 1500 to 2000 pounds, so 2 or 3 lb/ins increase in spring rate is not going to change the time taken to get the carrier to the back of the extension by any appreciable amount, but, once the buffer bottoms out in the rear of the extension the velocity is zero, so all forward acceleration comes from the spring. Stiffer spring more force; more force, higher acceleration; higher acceleration, less time....

NO. You are not going to put a spring in an AR strong enough to alter "dwell time". The force from the pressure on the piston pushing it open is in the neighborhood of 400 pounds. the mass of the bolt, carrier, and buffer is about a pound, calculate the initial acceleration of the reciprocating mass if the spring load is 6 pounds or 12 pounds . . . it's about 1.5% difference, that is not going to make an appreciable difference in time to move the mass 0.325 inch . . .

Two different values?

[utahjeepr]

Not claiming to have the answers here, but just throwing in what I do know.

So the rifle length system was said to operate at 12,000psi when I first learned about the M16. It was one of those stupid learning tricks. 12k at 12 inches. Now the number I commonly hear is 15k for rifle and 26k for carbines. Gonna assume that is due to hotter ammo, heavier projectiles,...

So for your question. We'll say the gas port of the rifle is exposed at 15k. Gas travels through the gas port, around the corner, down the gas tube, through the carrier, and into the bolt carrier bore. Whole lot of variables there for volume, drag, temp, ... Does any of that matter? In my kindergarten level understanding of gas flow dynamics the answer is no, the pressure should be relatively constant in all areas of the gas system during the dwell period. I'm willing to accept that that is not completely accurate and that it is more complicated than that, but this is my "operating knowledge". What I don't know is what that "universal" pressure is. I would not be surprised to find out that there is not complete equalization to bore pressure but I'm not going to speculate.

All that said, I'm in for some skoolin' once the experts chime in. Considering that Clint whom you quoted is about as expert in AR gassing as it gets, I'll put my chips on his answer for the win.

[Clint]

What is the peak force produced by the bolt–carrier piston system? Searching for an answer I am finding conflicting results.

Peak gas port pressures are 10k-30k psi, depending on gas system length, but they drop off quickly during the cycle.

Gasses pass the port, travel down the gas tube and expand inside the actual piston where the work happens. AR Engineers call this the "cavity".

The gas port meters a relatively small amount of gas into the piston system, which knocks the pressures down significantly.

Cavity pressures average 1500 psi, with the peak approximately double that.



Some revisions are in order. Corrected items bolded.

The force pushing the bolt forward is the same as the force pushing the BCG rearward.

Here are a few round starting values to calculate it both ways:

A) The BCG with a mass of 0.5 kg accelerates to a speed of 5 m/s in 2 mili-seconds. What force is required?

B) The piston has an effective diameter of .43" and an average cavity pressure of 1950 psi. What force is exerted?

Method A
0.5kg*5m/s/(.002s) => 1250 KgM/s^2 = 1250 N = 281 lbf

Method B
F = area * PSI

Area = Pi*R^2
3.1415*(.43/2)^2 = 0.145 in^2

0.145 in^2 * 1950 lb/in^2 = 283 lbf


So for walking around numbers, the average piston force is 280 pounds, with a peak over 400 pounds.



To echo Lysander's points about the spring; the piston force is 20-40 times higher than the spring, so the spring will have no meaningful effect during the initial acceleration.

[lysander]

Carbine length with M855:


Rifle length with M855:


The black line labeled "Cavity/Key Interface" is the pressure of the gas entering the cavity, since it has to expand to fill the cavity there will be a considerable pressure drop.

The peak pressure in the cavity is a tad lower:



That quote from me is a typo, it is not the force of 2000 pounds but a pressure of 2000 pound per square inch.

[Disciple]

Clint, thank you.

lysander, I don't see a great difference in peak pressure in those graphs. Somewhere around 2550 to 2700 psi if I am reading it right?

Why did you say 400 pounds (force) in one case and 1500 to 2000 in another?

[lysander]

That quote from me is a typo, it is not the force of 2000 pounds but a pressure of 2000 pound per square inch.

Depending on the ammunition, the weather, and the bullet weight, the pressure can be anywhere around 1500 to 2500 psi with the accompanying force.

Further reading:

The Gas Flow in Gas Operated Weapons

Sensitivity Study of Rifle Gas Systems

Comparison of a Theoretical and Experimental Study of the Gas System in the M16A1 Rifle8

[Disciple]

Thank you.