someguy2800
Member
Let’s change it from hydraulics to pneumatics. Think of it this way. Say we have pneumatic air cylinders, one that is 2” diameter and one that is 3” in diameter. Both are attached to a scale that can let the piston move in and out and measure the force on it. Let’s also say both pistons have the same volume under them with the piston retracted, 10 cu inches.
Now we pressurize both to 1000 psi. While holding the piston in place. The force on the 2” piston will be 3140 lbs. (PxR2xpressure)
Because of its larger surface are the pressure on the 3” cylinder is 7065 lbs.
Now we shut the air valve so no pressure can get in our out and we allow the piston to stroke out 20”. As the piston moves down the bore the space behind the piston increases and so the gas expands to fill the larger space and pressure drops. Acounting for the extra 20” of stroke the smaller piston will now have 72.8 cu in of space behind it (62.8+10), and the larger piston will now have 151.3 cu in (141.3+10). If we ignore temperature changes the pressure inside the smaller cylinder will now be 137.4 psi, and the larger cylinder will have just 66.09 psi.
This is why large bore small case cartridges do so well in short barrels, they make the most of the pressure inside the chamber early in there travel down the bore, but they will loose steam quickly and will stop showing velocity gains earlier than smaller bores with the same case capacity because of expansion ratio.
Now as you think about the air space expanding as a bullet moves, add powder burn speed into that thinking as well.
Now we pressurize both to 1000 psi. While holding the piston in place. The force on the 2” piston will be 3140 lbs. (PxR2xpressure)
Because of its larger surface are the pressure on the 3” cylinder is 7065 lbs.
Now we shut the air valve so no pressure can get in our out and we allow the piston to stroke out 20”. As the piston moves down the bore the space behind the piston increases and so the gas expands to fill the larger space and pressure drops. Acounting for the extra 20” of stroke the smaller piston will now have 72.8 cu in of space behind it (62.8+10), and the larger piston will now have 151.3 cu in (141.3+10). If we ignore temperature changes the pressure inside the smaller cylinder will now be 137.4 psi, and the larger cylinder will have just 66.09 psi.
This is why large bore small case cartridges do so well in short barrels, they make the most of the pressure inside the chamber early in there travel down the bore, but they will loose steam quickly and will stop showing velocity gains earlier than smaller bores with the same case capacity because of expansion ratio.
Now as you think about the air space expanding as a bullet moves, add powder burn speed into that thinking as well.