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Author Topic: 8mm BB FPS rules  (Read 3610 times)

Offline PissAunt

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8mm BB FPS rules
« on: April 03, 2005, 09:29:51 PM »
hey ive looked through the rules a billion times but didnt see anything for 8mm bbs like in the Marushin maxi carbine cuz im looking to buy one.
« Last Edit: December 31, 1969, 05:00:00 PM by PissAunt »
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Offline azsarge

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« Reply #1 on: April 04, 2005, 12:59:20 AM »
400 fps with .20 gram bbs

Translate that to whatever weight bb you will be using and voila.
« Last Edit: December 31, 1969, 05:00:00 PM by azsarge »
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Offline PissAunt

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« Reply #2 on: April 04, 2005, 01:29:11 AM »
its a single shot gun wouldnt 500 fps be allowed
« Last Edit: December 31, 1969, 05:00:00 PM by PissAunt »
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Offline Greg

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« Reply #3 on: April 04, 2005, 02:54:44 AM »
Quote from: "PissAunt"
its a single shot gun wouldnt 500 fps be allowed


When you start using the 400-500 fps range you have two extra rules:
-semi-auto only
-50' minimum engagement distance

That means you will almost definitely need a backup weapon, as most firefights are within 50'.

http://www.airsoftarizona.com/rules.htm
« Last Edit: December 31, 1969, 05:00:00 PM by Greg »
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Offline azsarge

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« Reply #4 on: April 04, 2005, 10:47:15 AM »
Quote from: "PissAunt"
its a single shot gun wouldnt 500 fps be allowed


In addition to what Greg posted, let me add this; The single shot rule was implemented with bolt-action rifles in mind.  Most of us fire in semi-auto anyway, so you would basically have a slight advantage in fps.

But, rules are rules, so if it IS indeed semi-only, you can use it.  However, it must be inspected by an AA member (used to be Paco, but he isn't around much) before you are allowed to use it at 500 fps at an official AA game.
« Last Edit: December 31, 1969, 05:00:00 PM by azsarge »
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Offline PissAunt

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« Reply #5 on: April 04, 2005, 11:40:36 AM »
yeah the marushin carbine is a single shot only, but i still dont know how to calculate the fps, is there a formula to use??
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Offline andyhinds

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« Reply #6 on: April 04, 2005, 11:56:03 AM »
Quote from: "PissAunt"
yeah the marushin carbine is a single shot only, but i still dont know how to calculate the fps, is there a formula to use??


Don't calculate the FPS, crono it.
« Last Edit: December 31, 1969, 05:00:00 PM by andyhinds »
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Offline azsarge

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« Reply #7 on: April 04, 2005, 12:39:22 PM »
Quote from: "PissAunt"
yeah the marushin carbine is a single shot only, but i still dont know how to calculate the fps, is there a formula to use??


Well, consider it as a ratio.  Use your algebra.

400 fps for .20g

___ fps for .XXg

Do the math.  Replace the XX with whatever weight you will be using, and cross multiply.
« Last Edit: December 31, 1969, 05:00:00 PM by azsarge »
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Offline Greg

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« Reply #8 on: April 04, 2005, 02:22:44 PM »
Quote from: "andyhinds"
Quote from: "PissAunt"
yeah the marushin carbine is a single shot only, but i still dont know how to calculate the fps, is there a formula to use??

Don't calculate the FPS, crono it.


While it's not an official chrono that we will use at a game, you can get a good idea of your gun's velocity by shooting a coke can at point blank. I'm not sure if this table will work for 8mm bbs, but here it is. From redwolf's site:

-ONE SIDE (BB remains in can after penetrating one side) 290 to 310 fps
-BOTH SIDES (BB makes it straight through and exits on the other side) 350 to 370fps
-BOTTOM (CENTRAL) 430 to 450fps
-BOTTOM (EDGE) 450 to 470fps
-TOP (EDGE) 480 to 510fps
-TOP (CENTRAL) 520 to 540fps

I've noticed that this table is a bit too high. I tested chris' SR15 after changing the spring using this method and it could punch through the bottom edge pretty well. However, he chronoed about 420 on a real chronograph. If you use heavier bb's, then I think the velocities on the table drop.
« Last Edit: December 31, 1969, 05:00:00 PM by Greg »
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Offline Cochise

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« Reply #9 on: April 04, 2005, 02:54:34 PM »
Quote from: "azsarge"
Quote from: "PissAunt"
yeah the marushin carbine is a single shot only, but i still dont know how to calculate the fps, is there a formula to use??

Well, consider it as a ratio.  Use your algebra.

400 fps for .20g

___ fps for .XXg

Do the math.  Replace the XX with whatever weight you will be using, and cross multiply.


There is actually a bit of physics here.  What the FPS gives us is a measure of kinetic energy (joules).  The formula for KE = .5 mv^2.  The squared term makes it not quite so cut and dry.  We can simplify this a bit, but you'll need to know how to use the square root button on your calculator.

1 divide .20 by the weight of your new BB weight.
2 multiply that number by 160000
3 take the square root of what you got

Example:
1 .20 / .25 = .8
2 .8 * 160000 = 128000
3 128000^.5 (square root) = 357 (rounded down)  This is in FPS

Similar formulas were used at LC3 to determine FPS values for different BB weights.

Using Christian's method, you'd get 320.  Well in the safe zone, but unnecessarily hindered.  Still requires algebra though.

8mm BBs come in different weights, usually not .20g, so it is not as easy as just chronoing it.
« Last Edit: December 31, 1969, 05:00:00 PM by Cochise »
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Offline PissAunt

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« Reply #10 on: April 04, 2005, 03:04:37 PM »
yeah in doing that math i get the fps with a .34 which is the lightest 8mm bb i could find and i got 306 fps.
« Last Edit: December 31, 1969, 05:00:00 PM by PissAunt »
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Offline Cochise

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« Reply #11 on: April 04, 2005, 03:52:23 PM »
Quote from: "PissAunt"
yeah in doing that math i get the fps with a .34 which is the lightest 8mm bb i could find and i got 306 fps.


...which for simplicity, you may want to round down to 300 or 305.

I forgot to mention, that formula gives an FPS for AEG rules.  If you are doing a sniper thing, you would replace 160000 with 250000.  

However, you will still want to coordinate a demonstration with the game organizer to show that no harm is done.  I think Arnie's Airsoft has the best method for this.  Basically, you give your gun to the organizer, walk 50' and the organizer shoots you.  Based on your reaction or injury, you may or may not be able to use the gun.  Basically, you don't shoot anything that you wouldn't be shot by yourself.
« Last Edit: December 31, 1969, 05:00:00 PM by Cochise »
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Offline Cochise

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« Reply #12 on: April 04, 2005, 03:58:01 PM »
For things such as this, someone may want to invest in getting a chronograph that measures Joules.  I'm not sure if there is a projectile-weight setting on these, but that would simplify things a lot.
« Last Edit: December 31, 1969, 05:00:00 PM by Cochise »
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Offline Farslayer

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« Reply #13 on: April 04, 2005, 04:29:30 PM »
It's not about FPS, like they are saying, it's the Joules that count...
« Last Edit: December 31, 1969, 05:00:00 PM by Farslayer »
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Offline Greg

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« Reply #14 on: April 04, 2005, 06:48:16 PM »
« Last Edit: December 31, 1969, 05:00:00 PM by Greg »
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