FPS conversion chart

[D.Smitty]

OK, lets hope that my new photobucket account works.  This chart was made assuming that a given gun will transfer the same amount of energy (same Joules) into a bb of any mass.  All I've got is MS paint, so I apologize for the crudeness of this chart.



I used conservation of energy:  squared the velocity of the .20 g, multiplied it by the ratio of 0.20 to whatever mass was in the column, then took the square root of that.  Can somebody check my math?

[jarhead904]

i fail at math

[D05GTO]

Looks good as a rough guide. Terminal performace would really show a better picture of what the different weigh BB's can do.  Ballistics have a lot more details.  Usually on ballistic charts you have to know the projectile coefficient (BC).  What is a 6mm BB?  No idea but I'm sure it's very very low. 

Also, these are muzzle velocities.  Biggest factor is the distance to impact and are very different for different weights in bb's.  Just a made up example....Say a .10 leaves the barrel at 400fps and at 10 feet it's down to 300fps.  A .20 may leave the barrel at 320fps and at 10 feet it will only be down to 300 fps.  I noticed this is really pronounced with .12's at about 50 feet the BB looks like it's just floating in the air with no velocity at all. 

Just some idea's.

D

[IronChef]

insurance only goes by fps. and fps only decideds MED...

[D.Smitty]

Looks good as a rough guide. Terminal performace would really show a better picture of what the different weigh BB's can do.   Ballistics have a lot more details.  Usually on ballistic charts you have to know the projectile coefficient (BC).  What is a 6mm BB?  No idea but I'm sure it's very very low.  

Also, these are muzzle velocities.  Biggest factor is the distance to impact and are very different for different weights in bb's.  Just a made up example....Say a .10 leaves the barrel at 400fps and at 10 feet it's down to 300fps.  A .20 may leave the barrel at 320fps and at 10 feet it will only be down to 300 fps.   I noticed this is really pronounced with .12's at about 50 feet the BB looks like it's just floating in the air with no velocity at all.  

Just some idea's.

D

Oh, I agree.  But I should point out that these calculations are estimates of muzzel velocities, which are used to classify airsoft guns and assign them MEDs.  Because so many people use heavier bbs to get a more stable trajectory, a lot of people are going to the chronos with 0.25s and 0.30s, etc, and the people running the chronos should have a handy chart to use for the sake of MED.

I'm not attempting to estimate the velocities of bbs at any range from their guns.  You are correct in stating that the terminal velocity of an airsoft bb in normal air is very low (because the drag force due to the viscosity of air is more significant), and will make it drop after a few feet (faster for lighter bbs).  

The next step would be to approximate the drag coefficients (related to what you would call the projectile coefficient) of these bbs at the different weights.  This could be done with simple measurements, but the problem is that there are so many variables that vary from gun to gun and bb to bb in addition to the drag.  

For example:  the force propelling the bb isn't contained in the bb itself, like in a bullet or shell, so the method of propulsion (as in strength of spring and shape of the air pressure wave from the nozzle of the piston head) will also affect how the bb accelerates, and that varies by a considerable amount from gun to gun.  

Now we come to the biggest variable of all:  the hopup.  This takes advantage of the strong effect air can have on the bb to create an area of low pressure above and high pressure below the bb, literally creating lift (like an airplane's wing) that slightly counters the effects of gravity for a short while.  The manufacturer of a hopup and hopup chamber are a source of even more variation.  

I'm not preaching, nor am I mad or anything like that.  I'm just saying that there are too many variables on airsoft bbs for me to personally account for on my own.  I'd need more guns (with interchangeable sets of parts and the ability to remove many of the externals so that the propulsion could be directly observed), multiple bbs types and masses, a very accurate scale (I would prefer it to be accurate to 10 micrograms) multiple chronos, high speed cameras (just becuase), a schlieren system attached to those cameras (if you've seen these, you'd want one too), a mass spectometer (to compare the makeup of different bbs), somebody who can use a mass spectometer properly, an accurate way to measure wind velocity (or a setting where I can control the air, like a wind tunnel) and a way to measure air pressure (which affects the drag, too).  Even after all that, a gust of wind would completely throw off the data.  

Believe me I WISH I could conduct such research.  Somebody get me a DoD grant!

[Straightshot]

Instead of using a slide rule, sun dial and a wish, the person chronoing can just have a bag of .2s, to ensure all fps is accuartely measured.  This has worked many times before. 

[GrimWulf]

Instead of using a slide rule, sun dial and a wish, the person chronoing can just have a bag of .2s, to ensure all fps is accuartely measured.  This has worked many times before. 

Whole heartedly agree!

[Dominum]

Instead of using a slide rule, sun dial and a wish, the person chronoing can just have a bag of .2s, to ensure all fps is accuartely measured.  This has worked many times before.   ;)

So brilliant, it could almost work!

+1

[D.Smitty]

Instead of using a slide rule, sun dial and a wish, the person chronoing can just have a bag of .2s, to ensure all fps is accuartely measured.  This has worked many times before.   ;)

You think I hadn't thought of this, already?  Guess who has the ammo to spare for this for a chrono: ME!

[Arbee]

Nah, it's too easy.  They'll never go for it.  You'd best bring your sextant, just in case something goes horribly wrong and we have to find the coordinates of the bb in question.

[Dominum]

BTW, the HGC have decided that .20s will be provided at chrono from now on, so this is essentially a nonissue.

That being said, the conversion chart in it's own right is still useful Smitty, I am going to sticky it, and put it in the tech section .

[IronChef]

if you are bored do an 8mm chart