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Old November 4th, 2008, 04:05   #29
Delierious Designer of Dastardly Detonations
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Join Date: Dec 2001
Location: in the dark recesses of some metal chip filled machine shop
I would think that at close ranges pellet mass would not affect the damage/pain potential of an AEG. AEGs have shown themselves to be constant energy devices endowing a pellet with a consistent amount of kinetic energy despite varying pellet mass.

However I've found that the first 20' of flight can incur a significant velocity loss, especially with lighter pellets (shooting at a chrony at range).

I suspect that this is due to two causes:

-air resistance roughly varies with the square of velocity
-lighter pellets have lower energy density in terms of Joules/(fps)

A lighter faster pellet will experience significantly higher air resistance to a slower heavier (equivalent energy) pellet. That increased energy loss due to higher squared velocity will be compounded by the greater fps reduction when the pellet sheds kinetic energy from it's lighter body.

I have found GBBs and NBBs of many types to be significantly non constant energy devices, especially if they are propelled by a 2 phase compressed liquid-gas method (propane or HFC134a). I suspect that this is due to several factors:

-Higher initial pressures
-Higher gas density (propane (1.5x higher) and especially HFC134a (3.5x) have higher density than air)
-More complex pneumatic arrangement

Room temperature saturated propane exerts around 115psi. Conversely an AEG springs peak force results in a pressure of only 20psi (guesstimate based on the spring exerting 15lbf at full draw). The considerably higher pressure will experience a much higher initial flow rate which will incur much higher pressure losses over the convoluted pneumatic arrangement typical to most gas powered guns. I surmise that flow rate dependent pressure drops over the various valves and openings cause a considerable loss of pressure applied to the pellet.

For awhile I bashed my head against the question why such a high pressure didn't result in deadly velocities. If an AEG spring exerted such a small pressure in comparison to the saturated gas pressure of propane, why weren't propane guns dangerous?

First off, long barreled propane propelled rifles can exert very high velocities (500+ fps easily). Secondly, a magazine contained a relatively limited volume of pre expanded gas. Propane requires significant energy to boil off and maintain mag pressure so I suspect that the phase of pellet propulsion relies almost completely on the expanded gas.

This fixes the supply of gas phase propellant so you don't get a tremendously fast BB because the pressure starts to rapidly drop as gas expands through the gun and experiences pressure drops over all the various valves and orifices.

Propane and HFC134a have significantly higher density than air. Heavier gases experience higher pressure drops over flow resistances which will reduce the final pressure applied to a pellet. These pressure drops over flow resistances (valves and ports) will be dependent on the flow rate of the gas and apparent backpressure provided by the pellet. This means that the gas flow rate out of a gun is significantly dependent on the resistance to gas flow that a pellet presents. Therefore the pressure drops over various flow resistances will be ultimately pellet dependent.

I surmise that a funny issue of series resistance to gas flow situation arises in a gas airsoft gun. You start with a high initial pressure in the mag until the mag fire valve opens. Gas flows through the fire valve and mag rubber and nozzle body until it impinges on the back of the bb and starts it moving. Pressure builds up behind the pellet and gets it moving.

Even if the gas is dense compared to air, it's definitely not dense compared to the pellet so the gas flow starts to become dependent on how fast the pellet presently moves in the barrel.

I suspect that the pellet represents a resistance to gas flow which is in series with all of the other orifice resistances. Heavier pellets represent higher resistances to flow while lighter (faster accelerating pellets) represent a lower resistance.

This is the bit for DonP: This presents a funny series resistance situation analogous to an electrical series resistance where a fraction of power is dissipated over all of the series resistances. For the most efficient power delivery to the intended load (the pellet, or analogous motor in your circuit) you should maximize the resistance of the intended load to put most of the pressure (or analogous voltage) drop at your intended load.

In the case of lighter pellets, the pellet moves fast enough that it allows a higher flow which incurs higher pressure drops over the orifices and valves resulting in lower pressure applied to the pellet. Less pressure means less force for given distance which means less muzzle energy. Heavier pellets slug down the flow rate and put more of the pressure drop at the pellet which means higher muzzle energy.
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