Help - Search - Members - Calendar
Full Version: FPS and BB Weight relation/ratio
Airsoft Forum > AirSoft Replicas, Tech Talk and Advice > Sniper Rifles
desmin88
Say I'm getting 500 FPS with a .2G BB, so about 2.31 Joules.
Now I use a .3G BB, is there a ratio or way to figure out mathematically what FPS I would be getting now, using the previous information of 2.31 Joules with a .2G BB?

I know this would be approximate and not an exact measurement, but I know theres a way, just I don't know enough about physics to do this.

Thanks
sniperx2s
QUOTE (desmin88 @ Dec 31 2010, 09:07 PM) *
Say I'm getting 500 FPS with a .2G BB, so about 2.31 Joules.
Now I use a .3G BB, is there a ratio or way to figure out mathematically what FPS I would be getting now, using the previous information of 2.31 Joules with a .2G BB?

I know this would be approximate and not an exact measurement, but I know theres a way, just I don't know enough about physics to do this.

Thanks

you could just use a FPS calculator. google it. pretty simple to use and way more straight forward than me giving you a mathematical forumula
desmin88
Actually, I want the formula. Call me weird.
Tux
The formula demands:

KE = Energy in Joules
m = Mass in Kilograms
v = Speed in Meters per Second

So we have a 0.20 gram BB which converts to a 0.0002kg BB.
We have a speed of 600 feet per second which converts to 182 meters per second.

Our KE=(1/2)(0.0002)(182^2).
desmin88
Assuming that we have

1/2(.2)(500^2)=1/2(.3)(v^2)

Simplify down we get

v = Squareroot of (.2/.3)(500^2)

So 250000 * .2 = 50000 / .3 = 166666.

Square root of that = 408 FPS approx. Am I correct here?
Brute71
sniper rifles tend to see energy increases as the BB weight goes up to a certain point, so what you calculate probably will be under what you will actually get.

I will use my CA M24 as an example of this, with .2g BBs my gun chrono's around 530fps (or 2.61 J). Now, the math states that I should be getting 433fps with .3g BBs but when I chrono my rifle with .3g BBs I get 451fps (or 2.84 J). I havent tried out any heavier BBs yet, but I should see energy increases with even heavier rounds (.36 to .40 might be the sweet spot for max energy). There really is no formula to determine the energy increases you will see (you could try and derive one but that would be a ton of work).

In theory you should get 408 fps on .3g BBs, but in reality you will probably get closer to ~420fps. I would recommend getting yourself a chrono so you can see for yourself what you are actually getting for this rifle and future replicas.

Here's an online MED calculator that does FPS and energy: http://msed.bbbastard.com/index.php?option...3&Itemid=57
desmin88
Thanks for answering my question brute71.
yee245
Brute is right about there not being an exact formula with what you will actually see as a velocity when you change BB mass, but if you want to find out the approximate velocity, or just calculate velocities at equal energy levels, it's a pretty easy conversion. You don't even need to calculate energy.

Here's my formula: v2 = v1 * √(m1/m2)
m1 = original mass (the mass you measured your BB at)
m2 = new mass (the mass you want to convert the fps reading to)
v1 = original velocity (the velocity you measured)
v2 = new velocity (the velocity that you are trying to calculate)

with the derivation here: http://www.airsoftforum.com/board/FPS-conv...&p=18246289

Pretty much, when I want to run some approximate numbers it's just a matter of if I know the velocity is going to go up (due to decreasing the mass) I'll divide the heavier weight by the lighter weight, take the square root (or if you're using a calculator that doesn't have a square root function, like the scientific version of the built in MS calculator, take it ti the .5 power) and then take that number and multiply it by the original velocity. if I know it's going to go down, I just divide the other way (or if I know I screwed up in dividing, I can just divide 1 by that number). As the formula implies, the velocity is basically proportional to the square root of the ratio of the masses, of course assuming equal energy.

Then, with real world values, you might have a higher-than-calculated or lower-than-calculated based on a variety of factors, one of which can include BB size. Smaller BBs, like say Bioval .27s or Excel bio BBs, are going to have less of an increase, if any, and a larger BB like an SGM will have more of an increase. Then, play with hop up, the tightness of the barrel, etc and the only way to know your actual velocity is to measure it.
This is a "lo-fi" version of our main content. To view the full version with more information, formatting and images, please click here.
Invision Power Board © 2001-2014 Invision Power Services, Inc.
CommunitySEO 1.2.3 © 2014  IPB SEO Module