FPS, range, and BB weight

[Agent47]

In an Airsoft GI video about the SOCOM M82, the speaker says he got around 390 fps with 0.2 g BBs, but 290 fps with .43 BBs...

 

I'm not a physics guy, but with heavier BBs, in order to achieve more effective range, would you need a more powerful spring to achieve the same fps as a spring pushing a 0.2g BB?

 

(on the downside, I'm pretty sure that heavier BB + higher fps = more ouch at close range...)

[Azulsky]

Not true. There are many forces acting on a bb during flight. BB weight positively affects many of them. Specifically in the notion that your range and accuracy are the same thing when it comes to ballistics. If you judge a torso sized grouping to be your effective range then increasing your accuracy extends the range at which you can achieve that grouping. A heavier bb is less effected by wind due to its increased mass, therefore your accuracy goes up, and your range does as well. Also in airsoft you take in to consideration the time to target of a round due to the slow speeds(relative to a real bullet) so there is a point where your gun is as effective as it needs to be.

 

There is a happy medium at which too heavy of a bb will simply not fly as far, gravitational acceleration is at constant exponential growth while the backspin needed to resist it degrades over time.

[Vlado]

How would a heavier bb mean more FPS? It makes no sense...

 

Are you faster holding 2 pounds, or faster holding 50 pounds?

 

A heavier bb does usually mean more range, but there is such a thing as too much.

 

Also no offense, but its common sense, not physics. You would obviously need a higher spring to get the same fps with a .43 as you would with a .20... but whats the point? People adjust the fps of their gun to the max the field allows, which is always tested using a .20.

[Hedganian]

I am a "physics guy", or at least my degree would seem to indicate this, and you'll be pleased to know that it looks like you've got the basics fairly right.

 

Do you want the simple explanation, or the detailed one? I'll give you the simple one first, then the details, and if you get bored, try not to let your snoring disturb the rest of the class....

 

The key here is ENERGY, not velocity - the energy will be the same in both cases (well, near enough) because the spring is the same, so if you increase the mass of the BB, the muzzle velocity will go down, if you use a lighter BB it will be faster. The advantage of a heavier BB is that it is less affected by wind and things once it's left the barrel, helping it to be (a bit) more accurate. The disadvantage is that it goes slower, so may not travel as far. It's swings and roundabouts here, you can't have it both ways.

 

The equation that relates to all this is a simple one - Energy (in Joules) = 0.5 x mass (in kg) x (velocity (in m/s) squared).

 

0.2g = 0.0002 kg (just in case people didn't know that).

 

From this you can work out the muzzle energy of any BB if you have its speed at the muzzle and you know its mass.

 

390fps with 0.2g ammo means a muzzle energy of about 1.41 Joules. 290fps with 0.43g BBs equates to 1.61J. Aha, I hear you cry, you said the energy will stay the same, but it's not the same is it? No, it's not. Want to know why? The reason is due to acceleration and inertia. The heavier BB has more inertia - resistance to movement - and so it accelerates slower. Therefore it spends more time in the barrel so it ends up getting more benefit from the "push" of the piston, reaching a higher energy by the time it gets to the muzzle. The opposite is true for a lighter BB - The same 1.41J of muzzle energy would make a 0.12g BB leave the muzzle at 504fps!

 

The same thing is true for longer vs short barrels - the same BB will end up going faster from a longer barrel than a shorter one assuming everything else is the same. Up to a point, then the barrel gets just too long and ends up slowing the BB down again. But that's another lesson.

 

Does that solve your problem? Feel free to ask any further questions. Also, you may find this page helpful.

[Agent47]

ok...wow I feel stupid, this is what studying anatomy late at night does to your brain...

[Hedganian]

Don't know why you feel stupid, you'd got the basics fairly right.

[Stealthbomber]

Just to add to the confusion...

 

Heavier BBs will usually "add power".

 

Whut?

 

You take a gun that is shooting at 330fps with 0.2g BBs. That's 1J muzzle energy.

Bung some 0.3g BBs in the mag and maths tells us that the gun should shoot those 0.3g BBs at 270fps.

Thing is, they won't.

They'll probably be shooting at around 280-290fps.

 

This is what's known as "energy creep".

I guess it suggests that lighter BBs aren't as good at making use of the potential energy of the spring.

[Hedganian]

Yep, covered that already.

 

 

390fps with 0.2g ammo means a muzzle energy of about 1.41 Joules. 290fps with 0.43g BBs equates to 1.61J. Aha, I hear you cry, you said the energy will stay the same, but it's not the same is it? No, it's not. Want to know why? The reason is due to acceleration and inertia. The heavier BB has more inertia - resistance to movement - and so it accelerates slower. Therefore it spends more time in the barrel so it ends up getting more benefit from the "push" of the piston, reaching a higher energy by the time it gets to the muzzle.

 

[greg]

Yep, covered that already.

You phisics guys are all the same.

 

I have a physics mate, granted, his 'Physics & Laser tec' degree is 20 yrs old & from Hull Uni, but he contracts to Nasa, so I usually give him the time of day.

 

I asked him the exact same question about the heavier bb giving more energy than predicted & he gave exactly the same answer you did.

 

He even went on to say how much more pronounced this would be, in longer barrels matched with cylinders, having a larger volume than required. VSR's, GBB pistols,,,,,,,,,,,,,,,,,,,,,

 

& then I got lost.

 

 

Greg.

[Hedganian]

So your point is......?

 

That some bloke who works for NASA agrees with my understanding of how physics works in airsoft guns?

 

That you don't understand physics?

 

[greg]

So your point is......?

 

That some bloke who works for NASA agrees with my understanding of how physics works in airsoft guns?

 

That you don't understand physics?

 

 

No, mate. No point.

 

Just an observation.

 

 

Greg.

[Hedganian]

No worries.

[noodels]

This is why here in Gothenberg Sweden (VSAF) , we crono ALL guns with the BB you intend useing & hoppup setup for the game.

Med is worked out after that. If you change BBs ( .20 - .25 , .25 - .28 ) it's back to the crono for you, to check fps & work out new med from this chart

[Hedganian]

That's not necessary, the standard of rating airsoft guns with 0.2g BBs across the board ensures an even playing field and makes chronoing a lot less complex...

[noodels]

but is flaud.

I have seen aeg's that cronoed 350 with .20's & 315 with .28's, my not seem much but you will feel the differance.

played a CQB game @360 fps with .20's no probs changed to .25's & people started to complain ( hits breaking skin ).

The higher the fps the more pronounst this will become. So in the UK your ok, but in parts of europe & the US things are differant

[Kuutti]

The chart makes sense to me, but I don't see how chronoing all guns with .2s is flawed. The whole constant weight thing and all that.

 

 

[kojak]

Just to add to the confusion...

 

Heavier BBs will usually "add power".

 

Whut?

 

You take a gun that is shooting at 330fps with 0.2g BBs. That's 1J muzzle energy.

Bung some 0.3g BBs in the mag and maths tells us that the gun should shoot those 0.3g BBs at 270fps.

Thing is, they won't.

They'll probably be shooting at around 280-290fps.

 

This is what's known as "energy creep".

I guess it suggests that lighter BBs aren't as good at making use of the potential energy of the spring.

 

It get's even crazier with gas rifles, presumably due to higher pressure building up behind a heavier BB as it remains in the barrel longer. My gbbr tested at about 2.5J with .20s and well over 3J with a .36 (WE m4, 262x6.025mm barrel, CO2 mag).

 

but is flaud.

I have seen aeg's that cronoed 350 with .20's & 315 with .28's, my not seem much but you will feel the differance.

played a CQB game @360 fps with .20's no probs changed to .25's & people started to complain ( hits breaking skin ).

The higher the fps the more pronounst this will become. So in the UK your ok, but in parts of europe & the US things are differant

 

 

You're talking about a completely different phenomenon. If you fired a .20 and a .40 from identical spring or electric guns, the force in joules at the muzzle will be pretty much the same. Since the heavier BB has more momentum, it will decelerate more slowly, and at a certain distance they will be going the same speed (although the .40 will take longer to get there). At that distance the .40 will transmit twice the energy.

 

While this doesn't matter much outdoors since pretty much no airsoft gun can hurt anyone at medium range (BB deceleration is pretty dramatic), I can imagine BBs would hit harder than expected at short distances. This is still not breaking any rules, but everybody knows how 'softers love to whine.

 

[greg]

It get's even crazier with gas rifles, presumably due to higher pressure building up behind a heavier BB as it remains in the barrel longer. My gbbr tested at about 2.5J with .20s and well over 3J with a .36 (WE m4, 262x6.025mm barrel, CO2 mag).

I think that's what my 'phisics guru' was getting at.

 

As said it got a bit misty at that point but he was adamant the effect would be more pronounced with longer barrels & more gas than is required for a .2. I guess the Tanaka's would be top of this pile. He seemed prety intent to blast on about gbbs.............

 

 

Greg.

[noodels]

I think the idea for the fps table in to retain a 1j inpact no matter the waight of the BB .

point in case , my Star L85 shoots 406fps (+- 2fps ) with .20's giving me a med of 10m & 380fps with .28's that gives me a med of 20m.

But the only way to know would to be to shoot each BB at each med & speed through a crono!.

 

[Hedganian]

If you fired a .20 and a .40 from identical spring or electric guns, the force in joules at the muzzle will be pretty much the same.

 

 

Wrong.

 

Read the earlier posts in this thread.

[kojak]

Wrong.

 

Read the earlier posts in this thread.

 

I did. Have you read the rest of mine? I'm not talking about the very small increase (5-10%) in muzzle energy you MIGHT get from a heavier BB making better use of the volume of air from your cylinder, I'm talking about the much higher energy (like double) you absolutely WILL get downrange as the heavy BB retains more of its inertia.

 

I say might because a theoretical AEG with the cylinder volume perfectly matched to the inner barrel volume will have no difference in muzzle energy with any BB weight. Extra cylinder volume means more time for positive pressure to accelerate the heavy BB, which is the "gas gun effect" all over again. No extra cylinder volume would mean total force is about the same no matter how long the projectile is in the barrel.

 

 

[TDS]

I did. Have you read the rest of mine? I'm not talking about the very small increase (5-10%) in muzzle energy you MIGHT get from a heavier BB making better use of the volume of air from your cylinder

then you are in the wrong thread, Also it changes from gun to gun, my AW50 had nearly a 50% increase in power from heavier bb's before I tracked down the problem, it is perfectly possible for a gun to be shooting magnum hard with light bb's but crono reasonably low, I won't go into more detail than that for obvious reasons but adjustable power supplies are the /least/ of field owners problems.

 

I'm talking about the much higher energy (like double) you absolutely WILL get downrange as the heavy BB retains more of its inertia.

 

So good deal, you can hurt people more efficiently?

Squeezing every last ounce of range should /never/ be a snipers goal, you should be going for accuracy, in which case you try a variety of bb's. You won't kill the target if he isnt where you were aiming by the time your OMGheavy bb gets there any more than if he/she didn't feel the hit so both points are fairly moot, Try out various ammos and decide what is best for you.

 

Virtually all reputable fields that I know of have started cronoing sniper rifles with heavy (.36+) bb's in order to stop dirtbags from taking advantage of power creep. I've always found that people running heavier ammo in automatics are easier to kill due to being able to get out of the way of the shots at reasonable range, or go prone before the shots reach you, and the slower shots are easier to backtrace to the source.

[Stealthbomber]

That's not necessary, the standard of rating airsoft guns with 0.2g BBs across the board ensures an even playing field and makes chronoing a lot less complex...

FWIW, I can kinda see the point of chronoing guns with the weight of BB the person intends to use in order to make sure that they don't creep over some legal threshold and end up shooting somebody with a "lethal" weapon on my site but it hardly seems worth the effort.

 

Having said that, if you're a site owner who's that determined to protect himself from any potential prosecution in law then you'll probably set a 1J limit for AEGs anyway and there's no way that an AEG is gonna creep up to 1.35J regardless of what ammo you use so any more complicated system is redundant anyway.

 

Besides, it's up to each player to ensure they're wearing kit that can withstand strikes from the most powerful gun on the site.

If a site allows 500fps sniper rifles but only 330fps AEGs you'd have to be daft to think "I'll be okay with some market-stall eye-pro cos I probably won't be hit by a sniper".

You've got to assume the worse-case-scenario which might mean that every sniper on the field decides to shoot at you simoult simultea simulaneuo at the same time.

On the assumption that your kit IS suitable for the most powerful possible strike, it really shouldn't matter is a guy who's AEG chronoed at 328 with 0.2s is now using 0.28s because the strike from that gun still won't be anywhere as dangerous as the strike from a 500fps sniper rifle.