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Chronoing with 0.2s

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Old May 14th, 2014, 16:10   #1
Huge
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Chronoing with 0.2s

Hey!

A question that has been bugging me for a while: why do we chrony all guns before games with 0.2s, especially bolt action rifles? Let's say a BA fires at 490 with 0.2s but at 400 with 0.3s. It will be ungameable on most fields unless you have a Level 2 cert. What is the reasoning behind this? Isn't it "safer" to fire at 390 with 0.3s than 450 with 0.2s?

Thanks!
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Old May 14th, 2014, 16:22   #2
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Kinetic energy.

Velocity doesn't hurt. Mass x Velocity hurts. If you set a velocity maximum of 400 fps with .2's, and your gun shoots 400 fps with .3's, the kinetic energy of the 0.3 is 1/3 greater than with .2's.

That's the short of it.

Here's a LARGE breakdown of it, with calculator:

http://msed.bbbastard.com/index.php?...d=53&Itemid=57
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Last edited by ShelledPants; May 14th, 2014 at 16:25..
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Old May 14th, 2014, 16:26   #3
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the purpose is not to chorno velocity.

It's to chrono MUZZLE ENERGY.

a .2 at 400 fps for example is 1.48 Joules.

a .3 at 327 fps is 1.48 Joules.

The problem with getitng people to chrono with the weight they are shooting is that unless you have a scale at the chrono station, you can't prove that the person claiming to have .3s in his gun are truly .3s.

It's easier to provide the ammo as a game organizer at the chrono station and ensure everyone is shooting 1.48J with .2s.

Well equipped game organizers could provide a heavier weight to cross reference the J output of a gun, especially if players intend to use heavier ammo... because you can start to see Joule creep in some setup. you can do a search for that, I'm not explaining that here. The TLDR is that the velocity/muzzle energy at the chrono is higher than the math suggests if you were to convert your .2 velocity/output to .3 output instead of measuring it.

As for YOUR attempt at interpreting it.

A .2 at 490fps is 2.22J
If that same gun were to chrono with a .3 mathematically it should be chronoing at 400fps to be at 2.22J

2 different velocities, 2 different weights, same muzzle energy.
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Old May 14th, 2014, 16:28   #4
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Some game/field/organizers, will chrono you with whatever BB weight and type you actually use, and rate by kinetic energy, which I believe is a better way to ensure no one uses over-power guns (or, more precisely, over powered combinations) compares to use 0.2g BB for all, especially with the number of GBBs nowadays. (For AEGs, the kinetic energy change among different BB weight is relatively small, whiles in GBB, the kinetic energy change is more significant. )

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Originally Posted by Huge View Post
Isn't it "safer" to fire at 390 with 0.3s than 450 with 0.2s?
390 with 0.3s = 2.12J
450 with 0.2s = 1.88J
In the end, it is the kinetic energy rather than the FPS that matters. Lighter BBs loses kinetic energy faster but to reduce complexity, only muzzle energy are being used most of the time.
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Old May 14th, 2014, 16:29   #5
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Quote:
Originally Posted by Huge View Post
Hey!

A question that has been bugging me for a while: why do we chrony all guns before games with 0.2s, especially bolt action rifles? Let's say a BA fires at 490 with 0.2s but at 400 with 0.3s. It will be ungameable on most fields unless you have a Level 2 cert. What is the reasoning behind this? Isn't it "safer" to fire at 390 with 0.3s than 450 with 0.2s?

Thanks!
Our club chronos at .20g because a) you don't have to reset the chrono(s) between players and b) the club supplies the chrono ammo and c) everyone thinks in "fps w/.20g" and not in Joules.

As for the safety aspect, BB Bastard did a study on muzzle energy vs. projectile weight vs. engagement distance: http://msed.bbbastard.com/
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Old May 14th, 2014, 16:32   #6
Huge
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Hmm I see. That's pretty interesting. Thanks for the input, I'll be reading more about this now.
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Old May 14th, 2014, 16:41   #7
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Additionally, Joule Creep is normally more prevalent in GBB guns because of the physics in expanding gasses rather than compression.

HOWEVER. you CAN create Joule Creep conditions in an AEG

For example my g36 has a 407mm barrel with a end ported cylinder. It's configured currently with a modify 120 spring that is shooting 380fps with a .2.

Using an FPS calculator to work out 380fps with a .2 I get 1.33J

If I work backwards from that math 1.33J with a .32 is 299.9 FPS.

The gun chronos between 305 to 310 with a .32 which is 1.42
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Old May 14th, 2014, 16:44   #8
Huge
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On a slightly different note, what is (if any) the usual leeway for fps discrepancy? Let's say someone comes in and chronies at 15-20 fps more than limit. Will he be allowed to play? Is it a significant enough difference? I have never seen it happen so Im curious.

Thanks!
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Old May 14th, 2014, 16:47   #9
Ricochet
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The heavier the round the more "kinetic energy" it has, meaning it hits harder. Velocity also comes into the equation. On 90% of airsoft, electric, spring, and most gas guns have a relatively consistent drop in FPS when introducing a heavier weight BB, keeping the kinetic energy, or joule limit, roughly the same. For instance, if your FPS limit was 420 with a 0.20g BB, your energy limit would be 1.64 joules of energy, and so one.

There are a few guns, mostly gas systems, that do not suffer from this FPS drop in the same way. So at most modern airsoft events, these guns are subject to chronographing with the weight of the BB they'll be using to ensure they are within the joule limit, and then cannot switch BB weights without re-chronographing.

"In a nutshell"

What generally happens with a gun that's over is this; there will be a field limit such as 400 FPS, which is the gold standard FPS for that fields performance. Then they'll have a safety window of 20 FPS in case of a fun that is "slightly" over, or a gas platform that may fluctuate in temperature change, creating a "hard limit" at 420 FPS. So in essence if you're a little over like 410, no big deal, but keep an eye on your gun. If you shoot 421 FPS however, your gun is not allowed. So for the sake of argument, if your gun shot 417 FPS, I'd seriously consider downgrading it, because a hot day could push you over the hard cap, which is the gun owners responsibility.
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Last edited by Ricochet; May 14th, 2014 at 17:17..
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Old May 14th, 2014, 16:59   #10
Comeau-SCS
 
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kinetic energy=0.5 m*v^2 you can calculate from that the conversion of different speed from that.

Quote:
Originally Posted by Huge View Post
On a slightly different note, what is (if any) the usual leeway for fps discrepancy? Let's say someone comes in and chronies at 15-20 fps more than limit. Will he be allowed to play? Is it a significant enough difference? I have never seen it happen so Im curious.

Thanks!
From my experience it's a hard limit. A rule is a rule. That's why most people try to get AEG around 380-390 to be sure it passes chrono.

Last edited by Comeau-SCS; May 14th, 2014 at 23:15..
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Old May 14th, 2014, 17:07   #11
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It's entirely up to the game organizer. I would hope though, if a greater majority of people were reading higher than normal that there could be exceptions to the rule. weather conditions can cause the guns to shoot higher from one day to the next, even AEGs.

I've seen as much as 20 fps difference on AEGs from one day to the next.

As much as 50fps from morning to afternoon on a kwa mp7 which is propane
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Old May 14th, 2014, 17:12   #12
Wrath144
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Comeau, you need to divide by two and then your kinetic energy equation is correct. Shelledpants, mass*velocity is momentum.
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Old May 14th, 2014, 17:22   #13
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Quote:
Originally Posted by Huge View Post
On a slightly different note, what is (if any) the usual leeway for fps discrepancy? Let's say someone comes in and chronies at 15-20 fps more than limit. Will he be allowed to play? Is it a significant enough difference? I have never seen it happen so Im curious.

Thanks!
Again the club I play with has FPS limits, not targets. Therefore it doesn't give leeway because you should really be under the limit anyway. Now if it was 1 or 2 fps, maybe that would be at the discretion of an admin on a case-by-base basis. 15-20? No way.

And I have seen it happen - guns hotter than the player thinks or often players won't know the muzzle energy of their own gun (it's new, they're new, etc) or weather changes push a near-limit gun over. They are usually surprised and then someone else lends them a conforming gun so they can play anyway.

This isn't universal - there are lots of groups and clubs with varying rules and guidelines that might allow it.
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Old May 14th, 2014, 19:24   #14
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Technically an FPS limit is a muzzle energy limit
when people say "420fps on .20s", that's a muzzle energy limit, not a speed limit.

Because of joule creep, being most prevalent in gas rifles, but also possible in AEG's, it's most effective to chrono a gun with the weight of ammo it will be using in game.
As opposed to chroning with .20s, ASSUMING the joule curve is linear, and allowing what is actually a hot gun into play.

For example, my WE PDW:
Shoots 380fps on .20s, which is 1.34j
But shoots 345fps on .28s, which is 1.54j, and also ABOVE the 1.48j limit that was in place at the time.

That's a short barreled gun, longer barreled guns can be significantly more drastic.
So don't be ignorant and think joule creep is a 20-30fps problem, because I've seen it go much higher.

It's really NOT difficult to chrono on a given weight.
Why the hell do you need to know the exact muzzle energy they are shooting? YOU DON'T.
All you need to know is that they're UNDER the limit. Stop making this complicated with conversion rates and airsoft calculators and all that useless time consuming crap.

This is a picture of my chrono, it has the maximum fps for a given joule limit of the most popular weights of ammo.
It's fast. It's effective. It's ACCURATE. And it works.


Yes, some people may lie about running heavier or lighter ammo than they actually have, but if you're chroning with .20s, then YOU as the ADMIN are potentially lying to yourself and everyone else because none of those muzzle energy readings are necessarily anywhere near accurate.
If you really don't trust your players, then chrono everyone on .28s. It's the most common ammo type next to .25, and it'll tell you for SURE if they've got a joule creep issue.

I've been through the accuracy, loopholes, and ways to cheat a chrono with many other very intelligent people. What we determined is that in order to be 100% safe, a field would need to provide multiple weights of top quality ammo in a different color for each weight, and then provide everyone with a gun.
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Old May 14th, 2014, 23:15   #15
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Originally Posted by Wrath144 View Post
Comeau, you need to divide by two and then your kinetic energy equation is correct.
Sorry forgot to write the factor, just corrected it. I dont pretend to rewrite basic physics ;-) The important thing I wanted to point out is that If a gun goes from 250 FPS to 500 FPS with same ammo it not 2 time more energy, it is 4 times.

Anyway it is better to calculate with the weight used as Thundercactus said. Also having double polish bb is crucial because i've seen difference between different BB.
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