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Do tightbores bring down the FPS?


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Old September 15th, 2011, 22:38   #31
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Join Date: Feb 2008
Location: North Delta, BC (Greater Vancouver)
Illusion and ThunderCactus both bring up good points. It may not be a bad idea to permanently post this info somewhere. A lot of countries actually use joules as their airsoft legal limitations. There is a direct connection between a BB's energy, and the gun it was propelled from. The basic formula being: velocity (speed) times Mass (weight) equals joules (energy). Other factors must be taken into account. Things like projectile aerodynamics, distance, gravity, and spin.

But illusion is correct. Joules rise when a heavier round is used. Even in a spring gun. In a GBB/GBBR you have to factor in gas expansion. For instance propane is compressed approximately 273 times, and has an expansion rate directly effected by temperature. Therefore the more time a projectile is plugging the barrel the more time it has to apply that force. Also as long as the gun hasn't finished it's cycle it may produce a larger amount of gas behind the BB. If the BB hasn't left the muzzle within this time then the gas will have more compressive force.
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Old September 15th, 2011, 23:51   #32
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Join Date: Jun 2002
Location: Toronto
Originally Posted by Ricochet View Post
The basic formula being: velocity (speed) times Mass (weight) equals joules (energy).
The understanding is correct. The formula isn't.

The formula should be:

(Velocity / 3.3)^2 x projectile mass / 2,000 = Energy

So for example, the energy of a 0.20g BB traveling 328fps is calculated as:

= (328fps / 3.3)^2 x 0.2g / 2,000

= 99.394^2 x 0.2 / 2000

= 9879.167 x 0.2 / 2000

= 0.988 Joules

Last edited by ILLusion; September 15th, 2011 at 23:56..
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Old September 16th, 2011, 04:03   #33
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lol @ people who use heavier ammo to try to get under the FPS limit
I absolutely agree with everything you said. A GBBR will get higher FPS simply because it's shooting with a variable amount of force (My PDW shoots 1.34j on .20s but 1.8j on .28s). But your FPS only affects your range so much...
Our limit out here is 400fps, and the 1.4j KJ's are still out ranging most of the 1.4j AEG's, even when both are using the same ammo. But dependent on the weather of course lol
From what I've seen on the field, a gas pistol with an absolute velocity of 270fps on .30g, will simply get better performance than an AEG shooting 270fps on .30g BBs. Same muzzle velocity, same ammo, one simply performs better than the other, and by a clearly visible amount.
Having more gas available to center the BB going down the barrel will absolutely help accuracy, but your range is dominantly controlled by your hop rubber.

Seems to me, you can't get the proper amount of hop-up in an AEG to produce the spin you need to use heavier ammo at long range. At the hop pressure you would need to GET that backspin, it would simply jam the gun.
So I would say, because a GBB has more pressure available (as you were saying) because you're getting more pressure with heavier ammo, it's able to operate with more hop-up pressure, and thus get more range with heavier ammo.

optimal hop = more range. And most GBBR's are able to get .30s to curve straight up, whereas you're lucky to get even a straight trajectory with an AEG shooting less than 350fps on max hop.

Last edited by ThunderCactus; September 16th, 2011 at 04:08..
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