FPS limits and BB weights

[Hawk]

Last night I was talking to one of my team members ( hippy ), and he brought up the topic of FPS limits and BB weights. Now correct me if I'm wrong, but arn't you supposed to chrono with a .20, no matter what weight BB your going to be using during the day? hippys argument was that you can chrono with the BBs your going to be using, .25, .28, .30 etc. Discuss

[Boo-Sabum Ben]

Readings should be done with a .2

Some sites will let you Chrono with a different weight and then do the maths on the spot to make sure you're still under the limit.

[Hawk]

Re: FPS limits and BB weights

Thats what I tried to tell him, but he paid no attention

[plainlazy44]

Re: FPS limits and BB weights

You can't measure the energy of a bb based on its speed.

F=Ma : If a bb has more mass but has the same acceleration then the force is higher but the chrono won't pick it up.

The amount of pain you feel from a bb impact is dependant on the force, not the speed.

Hawk, you are right and your friend is wrong, whatever he might say.

[Hawk]

Re: FPS limits and BB weights

Does it help that I know A level physics :D

[cannon_fodder]

Re: FPS limits and BB weights

A sensible site will supply the Mags and BBs for testing at chrono.

If everyone tests with exactly the same make and weight there can be no issues and no need to use convertion tables based on wieght.

snipers turn up with an empty mag and a marshall loads it with a speedloader of .20's even if the sniper is going to use .40s in the field


Any site that tests with player supplied BBs is asking for problems. You can be sure that some players with a hot gun would keep a mag of .40s so thay would pass chrono at 330 fps and run arround at 400 fps with .20s

[windywindmill]

Re: FPS limits and BB weights

Does it help that I know A level physics :D

Then you should have won that argument hands down!

[plainlazy44]

Re: FPS limits and BB weights

Yes. Slap him round the face with some science.

[Decho]

Re: FPS limits and BB weights

You can't measure the energy of a bb based on its speed.

F=Ma : If a bb has more mass but has the same acceleration then the force is higher but the chrono won't pick it up.

The amount of pain you feel from a bb impact is dependant on the force, not the speed.

Hawk, you are right and your friend is wrong, whatever he might say.

Much as overall you are right, your argument is, sadly, wrong. While F=ma is a personal favourite it doesn't help us calculate the energy of the round. This is done using KE=1/2mv^2, which does, of course, require the velocity.

The amount of impact damage is indeed dependent on the force however, hence why padded clothing reduces pain. But yes, I think that some kind of explanation of the fact that the point of the chrono is to measure muzzle energy and that therefore it must be done with a certain mass round should be distributed everywhere, it pains me that people don't understand it.

[tyreal]

Re: FPS limits and BB weights

The quantity you are looking for is kinetic energy. This is the energy carried by the projectile and is imparted during a collision. F=ma has no effect here as the BB is not accelerating once it leaves the barrel. The formula is E(subscript k) = 1/2mass x velocity squared. As this shows the mass of projectile is very important to it's kinetic energy. If BBs aren't all chronoed with 0.2g then they may be above the site limits. If anyone is doing this then they will find themselves in VERY hot water.

Edit: Ninja'd

[senselessg]

Re: FPS limits and BB weights

People used to cheat the chrono at my regular site this way. The marshals picked up on it and random chrono checked them. They came in over on all weapons and were only allowed to use their pistols

[No1_sonuk]

Re: FPS limits and BB weights

You can't measure the energy of a bb based on its speed.

F=Ma : If a bb has more mass but has the same acceleration then the force is higher but the chrono won't pick it up.

The amount of pain you feel from a bb impact is dependant on the force, not the speed.

Hawk, you are right and your friend is wrong, whatever he might say.

Like his friend, you are also wrong.
F=Ma is "FORCE", not muzzle energy. 2 different things.

If what you say is true, no one would feel a hit as the accelleration is actually negative (the BB is slowing down).

Muzzle energy (Ek) = 1/2 x mv^2 (v squared)
m in kg and v in metres/second will give the energy in joules.

328 fps = 99.9744 m/s 0.2g = 0.0002kg

Ek = 0.5 x 0.0002 x 99.9744 x 99.9744 = 0.999488 J


For a 0.25g BB @ 328 fps, Ek = 1.24936 J
Using that formula, to match the 0.2g BB @328 fps energy:
fps of a 0.25g BB would need to be 293.3719892
fps of a 0.28g BB would need to be 277.2104759
fps of a 0.3g BB would need to be 267.81078

If your local has a limit of 350 fps with a .2gBB:

0.25g = 313.049389
0.28g = 295.8038713
0.30g = 285.7737076

So you can see the difference from 0.2g to 0.3g for the same energy (1.13806224 J ) is almost 65 fps.


The Xcortech chrono I have can show muzzle energy as well as fps and rate of fire. You select the BB weight, and it calculates the energy based on the fps.

Edit: Multiply ninja'd

[Boo-Sabum Ben]

Not another science battle....

[oneshot187]

There is also an app on the iphone that will do all the calculations for you..

[happyal]

Re: FPS limits and BB weights

You should chrono your gun with the bb's you intend to use, and work out the energy level from that.