Question about FPS rules?

I am about to purchase my first AEG and have a question. I realize that most fields limit FPS to 400. If a weapon is rated at 400 fps but when I get it is firing a little hot...what will probably happen when my weapon is chrono'd? I plan to get an M4 variant rifle and know it will be a Hicap. Should I be conservative and purchase a rifle that is rated closer to 350fps to be safe? I'm thinking KWA M4A1 at this point, and it is rated 400+ fps.

What advise can you give?

Airdog rules are 420 fps...From what I understand .25 bb's will crone lower and after some use the stock FPS will decline on most AEG's

Somebody correct me if I'm wrong

dont chrono with .25s, it is basically the same as lying about your fps.

KWA's tend to start around 440 and then settle around 410-420

You can chrono with .25. My brother and trent did it all the time at George Town. They actually chronoed with .36's. But another factor that you also have to remember is that your starting fps is going to drop a little after your first use. Then it will drop at a steady rate. When I used my stubby for the first time, it chronoed at 375 to 380 fps. Now when i chrono at the airdog games, it has droped to 340. I have used my stubby about 7 to 8 eight game days. And now its staying around a constant 335. So the 335 fps is just about where my fps will sit. It of coarse will still drop every so often.

Im somewhat divided on this matter. If you chrono with .25s it will read as a lower FPS but wont it have a great energy because of the heavier weight? I can remember back to pyshics but wouldn't getting hit with a .2 bb at 50 feet with an airsoft gun hurts less than getting hit with a .25 at 50 with the same gun? Or will the drop in FPS from .2 to .25 even out the hurt?

At point blank the energy transfered by .2s and .25s is the same, but at range the .25 looses energy slower, hurting more.

a 415 fps gun might fire a .2 that in 100 feet is at 315 fps, whereas a .25 might still be at 340, even though it only came out at 380

my friends kwa m4 probably about after 5k rounds through it his was shooting 398 pretty consistently after that

Appreciate the info...so I should be able to drop to within acceptable limits at some point soon....so cool. I think I'll go for the KWA

Electioneer wrote:Im somewhat divided on this matter. If you chrono with .25s it will read as a lower FPS but wont it have a great energy because of the heavier weight? I can remember back to pyshics but wouldn't getting hit with a .2 bb at 50 feet with an airsoft gun hurts less than getting hit with a .25 at 50 with the same gun? Or will the drop in FPS from .2 to .25 even out the hurt?

force= mass (x) acceleration
So essentially if you use a higher mass you are going to have more force. But at the same time your initial velocity will be less. Because chrono is the standard at measuring the force of the bb (that's what really counts anyway) you are manipulating the results. So you essentially having a higher force then what you are reporting at chrono. Fields have limits for a reason. It keeps people safe, and it's their field with their insurance so they get to make the rules. If you are shooting hot then there should be restrictions to make sure that you are being safe (med, semi only, etc.) So if you chrono with a bb that has more mass to keep from shooting "hot" then you are still having a higher force, thus still in the unsafe zone. Fields will mostly work with you if you are shooting 10fps hot or so. I personally don't mind going home with welts, but there are kids that play this game, and they've got mothers. And if these mothers see their little boy (or girl) with a bruise because some dude shot him too close then there will be Hell on Earth. I don't blame the fields for not wanting that. Bottom line is talk with your regular field. As long as you are mature and not just blasting at people from 15ft away with a 500fps m4 on full auto then you will probably get the people who own the place to work with you. Just understand that then you have a magnifying glass on you. Just use your head and talk with the people in charge before hand. Some people are super strict when it comes to fps limits so if you chrono 401 and limit is 400 you're then a sniper and semi only. Just talk with them.

Thanks, that was the physics class that was eluding me.

NJSC wrote:

Electioneer wrote:Im somewhat divided on this matter. If you chrono with .25s it will read as a lower FPS but wont it have a great energy because of the heavier weight? I can remember back to pyshics but wouldn't getting hit with a .2 bb at 50 feet with an airsoft gun hurts less than getting hit with a .25 at 50 with the same gun? Or will the drop in FPS from .2 to .25 even out the hurt?

force= mass (x) acceleration
So essentially if you use a higher mass you are going to have more force. But at the same time your initial velocity will be less. Because chrono is the standard at measuring the force of the bb (that's what really counts anyway) you are manipulating the results. So you essentially having a higher force then what you are reporting at chrono. Fields have limits for a reason. It keeps people safe, and it's their field with their insurance so they get to make the rules. If you are shooting hot then there should be restrictions to make sure that you are being safe (med, semi only, etc.) So if you chrono with a bb that has more mass to keep from shooting "hot" then you are still having a higher force, thus still in the unsafe zone. Fields will mostly work with you if you are shooting 10fps hot or so. I personally don't mind going home with welts, but there are kids that play this game, and they've got mothers. And if these mothers see their little boy (or girl) with a bruise because some dude shot him too close then there will be Hell on Earth. I don't blame the fields for not wanting that. Bottom line is talk with your regular field. As long as you are mature and not just blasting at people from 15ft away with a 500fps m4 on full auto then you will probably get the people who own the place to work with you. Just understand that then you have a magnifying glass on you. Just use your head and talk with the people in charge before hand. Some people are super strict when it comes to fps limits so if you chrono 401 and limit is 400 you're then a sniper and semi only. Just talk with them.

+1 to this, he beat me to the explanation. Excellent reply bud.

The physics of it is this: Kinetic energy=.5(mass)*(velocity squared) While the velocity of the bb may be dropping, the mass of the bb is increasing, so the kinetic energy is essentially staying the same. The motor produces the same amount of energy in every shot, and it's the energy that causes damage and pain, not JUST the velocity.
The reason you are supposed to chrono with .20s is because you have to have a standard mass in order to compare the guns. So in reality, the field limit rule at airsoft is 420 fps WITH .20s. I'm too lazy to do the math right now, but if you worked that out to find how many joules of kinetic energy that equals, then you could use that formula and plug in the kinetic energy you just found and whatever mass BBs you are using to find the proper velocity.
You shouldn't chrono with .20s though, because the field limits for FPS are set at what are considered safe limits for a set weight of BB. To use an extreme example, if you get hit by a paper ball shot at 500 fps, it won't hurt, it's too light. However, if you get shot with a metal ball that's the same size at the same fps, it would be like getting shot with a bullet. Sure, you could sneak by a chrono by using heavier BBs, allowing you to play with a hotter gun, but it is more dangerous because the kinetic energy which is transferred to the body on impact is greater than it would be with a lighter BB at the same fps.

EDIT: Bah... I didn't realize somebody else had already posted an explanation. Oh well. Double the formulas, double the fun right? >.>

Yep. If desired, I can get into the lorentz transformation and explain how a lighter BB travelling faster actually has more energy than a heavier, slower BB because the faster object has a higher relativistic mass (when an object approaches c, mass increases by 1/gamma).

However, I think we should just let it rest.

I haven't enjoyed physics discussions so much since my junior year of college...but another question. I understand that the spring is what determines fps. Not knowing, at this point, what is involved in upgrading AEG's and such, would it be practical to have a "slower" spring on hand for lets say ~350fps for cqb and a heavier spring for field play ~400 - 420fps? I could just switch the springs in and out to achieve the desired/necessary velocities.

What say you?

Completely understandable. Most people have two guns though