Disciple
07-24-21, 20:50
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?
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?