FPS - an article

FPS - an article
(including online fps calculator)

Well I thought I'd get some of my thoughts on 'paper' about fps, and at the same time create a page that would be useful for your average airsofter.

Contents

FPS - so what does it mean ?

How can I calculate my airsoft replica's energy?

So how does extra FPS affect my range?

Useful links

FPS - so what does it mean ?

Forgive me, I'm going to cover the basics and then move on to, so if you are an experianced airsofter you'll already know a lot of this.

FPS stands for Feet Per Second, for those of you who hate anything other than SI units (Systeme Internationale), one fps is 0.304 meters/sec, (equivalently 1m/s = 3.28 fps). This is why 328fps is thrown around as a limit. 328fps is 100m/s, and a 0.2g bb travelling at this speed possess the energy of exactly 1 Joule.

Energy (in joules) = 1/2 mass * velovity^2

How can I calculate my airsoft replica's energy?

Using this handy form I have created for you below, you can calculate your replica's energy in joules. As a by-product it'll also give you your bb's velocity in m/s. To calculate the energy in joules, simply enter the mass of ammunition (in grams) that you use, and the fps that you've read from your chrono unit.

Input Values:

BB weight (in grams):
Speed (Feet Per Second):

Results: (rounded to 2 decimal places)

Velocity (m/s):

Energy (Joule):

FootPounds

You can also enter another number in the Energy(Joule) box, and click 'Convert' to get that figure in Foot-Pounds.

The generally accepted limits in the UK, are as shown below:

Type	Speed	Energy
AEG	328fps (with 0.2g BB)	1 Joule

These are of course self regulated, and you'll need to check with your skirmish site for what they accept as limits.

So how does extra FPS affect my range? Well I've been having a think about this, and we will be conducting some tests with some AEG's at some point in the warmer part of the summer, from these we'll be able to deduce some real-world figures. Right now though the easiest thing that I can do is calculate some range using basic physics, making certain assumptions. For starters we'll ignore hop-up, as we know it increases range, and I'm not going to sit here and work out higher level physics using specific air density and fluid dynamics - I'll do that later on =). This model assumes a curved flight path, I know hop-up produces a straighter path, but you'll be suprised how close the figures from this model match to real-life.

From basic physics, lets start with a basic equation:

s = distance
u = initial velocity
v = final velocity
a = acceleration
t = time passed

s = ut + 1/2 at^2

Assume you are firing the bb from a height of 1metre (i.e. with your rifle shouldered), lets calculate the time it takes for your bb to drop and hit the ground:

s = ut + 1/2 at^2 hence,

1 = 0 + (0.5* 9.81) * t^2
t = sqrt( 1/(0.5*9.81) )
t = sqrt(0.203...)
t = 0.45 seconds (2SF)

Hence time taken to fall 1 metre is 0.45 seconds. With a velocity of 328fps or 100m/s, this mean your bb will travel (328*0.45)=148feet or 45metres before hitting the ground, giving you your effective range. If you don't know the energy of your rifle, you can either calculate it above, or use 1J as the limit for AEG's, and 2.31J as the limit for single action.

You can use this small javascript form to calculate your theoretical effective range. Please note this ignores a lot of physical factors, and assumes a firing height of 1 metre off the ground, and assumes that you are firing over a level "range" .

Initial values

Mass (grammes)
Energy (Joule)

Results: (all to 2 SF)

Muzzle velocity

(fps)

(m/s)

Effective range

(feet)

(m)

Useful Links:

*SCI Journals Conversion Factors*	- a handy page that lists the Conversion Factors for SI and non-SI Units
*Hatchet's Guide to Airsoft*	- Hatchet's page covers a few points on FPS and the law, explains theses topics in plain English.
Practical Airsoft	- Redcap's site covers a wide range of aspects concerning airsoft and the law, and it contains some of the best researched articles in the UK.

Related Site Articles:

TBA

This page last updated: Tuesday, January 18, 2005 1:50 AM