The Joule and the Impact of the hit with heavier BBs

[BBS]

excellent post Brian. that post alone could be stickied.

[CDN_Stalker]

An example I experienced that shows roughly what Illusion said about gas guns is my KSC G19. It's upgraded, only fps enhancing thing is the tightbore (I do believe the mass of the metal slide and stiffer recoil spring causes a delay in the blowback, therefore letting the gas expand a bit longer, increasing the fps............. which seems to be the case in the KSC Mk23 recoil spring I rebuilt last night, differently, it has a lot more resistance now so the action is snappier, and seems to shoot harder as well) and I chronied it one day at 341fps with 0.20g BBs. I got that same reading both shots I took. Then I chronied it next shot with a 0.43g BB (what I was using full time back then) and I got about 265fps. I told Brian, he explained the increase in gas expansion thing he pointed out above, and figured that for the gas expansion with the heavier BB, it'd be roughly about a 0.20g BB coming out aroud 380fps.

Here's an interesting bit of info related to above. I study a lot of airsoft ballistics, largely in the area of sniper rifles. Numerous times when I chrony my or others guns with 0.20g BBs, I might follow up with a 0.36g if that's what the user wants to use. I continuously find a 100fps difference in velocity between the two. So if the rifle shot 341fps using a .20g, the 0.36g BB would sit around 240fps, and a 0.43g BB might be ~40fps lower in the same gun. With my G19, I only had a loss of ~75fps using the 0.43g, around half of what fps droppage I'd get with a constant air gun.

[Scarecrow]

When I go deer hunting with my .308, I make 168gr (grains) bullets. When I go moose hunting, I make 220gr bullets. I use a similar amount of powder for both. I use 220gr on a moose because its bigger and you need more kinetic impact energy at the target (lungs, heart area) to inflict maximum damage and kill shock. I'm no physicist, but to me from that experience, that same thing applies to airsoft ballistics, would it not? Ie: the heavier grain bullet imparts more kinetic energy on impact...

[Skladfin]

Nice post Illusion!

Sticky +1

[Nik12]

Originally Posted by Scarecrow

When I go deer hunting with my .308, I make 168gr (grains) bullets. When I go moose hunting, I make 220gr bullets. I use a similar amount of powder for both. I use 220gr on a moose because its bigger and you need more kinetic impact energy at the target (lungs, heart area) to inflict maximum damage and kill shock. I'm no physicist, but to me from that experience, that same thing applies to airsoft ballistics, would it not? Ie: the heavier grain bullet imparts more kinetic energy on impact...

That is true. But if the heavier grain bullet has a lesser velocity (even by a bit) it cancels the effect.

Knietic energy = (mass*veloctiy*velocity)/2

As you can see, veloctiy is squared in the relationship where as mass is not, so a slight decrease in velocity has more effect than a slight increase in mass.

However, as ILLusion said, it's only true for AEGs and Springers since they are 'constant'.

[Rob]

I suppose this gas gun variable velocity fact is the reason why the WA gigant has such large fps values even on duster; the long barrel means the bb will spend more time in there and thus more gas is expelled.

i can see how this works in a gbb gun, where the gas propels the bb first and then diverted to the blow back function via a valve thingy in the nozzle (WA design for instance), but does this work in nbb guns? the tanaka sniper rifle series has large fps numbers but is that the result of a valve with a set amount of gas coming out or is the gas output variable as in a gbb

[Flatlander]

Couple things to note:

The total kinetic energy is NOT just it's linear velocity (E=.5*m*v^2) but you must add its rotational energy also (E=.5*I*w^2) where I=moment of inertial and w=angular velocity. So with no way of determining the angular velocity (rate of spin) you can't accurately predict/calculate the total kinetic energy with just a chrony.

Momentum has more to do with how hard the bb's will hit (M=m*v) and isn't related to kinetic energy. Obviously a 25% increase in BB weight (.20-.25) does not result in a 25% decrease in velocity, therefore more momentum is present.

I can't comment on any empirical evidence (as I have none) but this is my knowledge of the theory.

[Nik12]

Originally Posted by Flatlander

Couple things to note:

The total kinetic energy is NOT just it's linear velocity (E=.5*m*v^2) but you must add its rotational energy also (E=.5*I*w^2) where I=moment of inertial and w=angular velocity. So with no way of determining the angular velocity (rate of spin) you can't accurately predict/calculate the total kinetic energy with just a chrony.

Although, would the rotational Kinetic energy be negligable compared to the linear kinetic energy?
What is the moment of Inertia for a sphere? I'm not entirely sure, but I'm assuming that it'll be similar to the moment of inertia of a circle in which the radius is to the power of four. 6mm (or 0.006m) to the power of four is a much, much smaller number (we are talking about several magnitudes of ten smaller). That probably makes Pi the biggest factor in the equation for angular/rotational energy. The smallest factor in the linear energy equation is mass (0.00025 for .25g BBs), but it is being multiplied by a much bigger number (ie the velocity squared).
Looking at it in that manner, I'd say Angular Energy << Linear Energy.

Originally Posted by Flatlander

Momentum has more to do with how hard the bb's will hit (M=m*v) and isn't related to kinetic energy. Obviously a 25% increase in BB weight (.20-.25) does not result in a 25% decrease in velocity, therefore more momentum is present.

Sure it is. If kinetic Energy is E=(m*v*v)/2 and Momentum is M=m*v then a simple relationship between Momentum and Kinetic energy can be defined consider both rely on the basic measurements of mass and velocity.
Example:
E=(M*v)/2 since M=m*v
So really Kinetic energy is really just momentum multiplied velocity then halved.
It makes sense that these two be related. Obivously as momentum increases you would expect the kinetic energy to increase. Obviously as Veloctiy increases you would expect both to increase. Obviously if the object became heavier you'd expect both the increase.

I agree that an increase in weight won't mean an exact decrease in velocity. Weight is absolute in this case. Aside from removing or chemically changing the composition of the BB, the mass will remain the same throughout the entire trajectory. So to begin with you know the BB is either .20g or .25 g (or what ever weight you want) and that won't change. Velocity is a much more fickle matter. Gun performance/consistancy, air currents/resistance, and all that play a big role in velocity. The effects of one thing or many things could (and most likely does) change depending on the density of the BB used. My guess is that because the velocity of an object moving through a fluid is largely based on it's shape, since the BBs are exactly the same shape, the velocity has a somewhat noticable decrease, but not that which is required to completely balanced to effects of the heavier BB.

Originally Posted by Flatlander

I can't comment on any empirical evidence (as I have none) but this is my knowledge of the theory.

I can't comment on any either (nor do I), but this is my knowledge of basic equation and relationship manipulation.

[ILLusion]

Originally Posted by Nik12

That is true. But if the heavier grain bullet has a lesser velocity (even by a bit) it cancels the effect.

You're right... again, AT THE MUZZLE. The muzzle energy WILL be consistent in an AEG or springer.

However, to reiterate, at range, a lighter BB loses much more of that energy due to other forces (namely, wind or standing air).

Have you ever shot BBs through a crosswind? Have you ever noticed how a lighter BB will always stray further than a heavier BB? Now imagine those two BBs shooting straight with no crosswinds, the wind they face head on will affect the lighter BB more than the heavier BB, the same way a crosswind will affect a lighter BB more than the heavier BB. This is the explanation for the comments people have made that "we're not shooting through a vacuum."

Originally Posted by Rob

I suppose this gas gun variable velocity fact is the reason why the WA gigant has such large fps values even on duster; the long barrel means the bb will spend more time in there and thus more gas is expelled.

i can see how this works in a gbb gun, where the gas propels the bb first and then diverted to the blow back function via a valve thingy in the nozzle (WA design for instance), but does this work in nbb guns? the tanaka sniper rifle series has large fps numbers but is that the result of a valve with a set amount of gas coming out or is the gas output variable as in a gbb

This effect is true on almost ANY gas gun - INCLUDING real steel guns. Once the propellant has burned off, the expanding gas is what pushes the bullet forward. Longer barreled guns will have higher velocities than shorter barreled guns due to the greater amount of time the gas has to expand and impart energy on the projectile.

Tanaka rifles have large velocity numbers as a result of long barrels combined with high gas outputs (depending on how wide your PCS bolt is opened or closed.) In wide open, the gas valve opens much wider than any GBB can produce.

The limitation of bolt action or any single-action gun for that matter, is that the gas is only fired for a set amount of time, regardless of the weight of the BB. There is no rocket/floating/reed/power valve to detect the release of the projectile and shut off the gas valve like in self-chambering guns. There is an upside and a downside to this. The upside being that the gas volume output is ALWAYS the same, regardless of the projectile weight. Consistency is key to sniping.
The downside is that there is a potential for inefficient gas use (too much expelled), or the opposite - not as much gas is expelled, which leads to a lower velocity than can be achieved.

[Flatlander]

Originally Posted by ILLusion

Have you ever shot BBs through a crosswind? Have you ever noticed how a lighter BB will always stray further than a heavier BB? Now imagine those two BBs shooting straight with no crosswinds, the wind they face head on will affect the lighter BB more than the heavier BB, the same way a crosswind will affect a lighter BB more than the heavier BB. This is the explanation for the comments people have made that "we're not shooting through a vacuum."

Correct and this is due to it's momentum not it's kinetic engergy.

Also, when I meant momentum and energy have nothing to do with each other I meant it's like comparing apples to oranges. I have yet to use (or see) the energy/momentum equations combined/manipulated like that (I'm a mechanical engineering student).

The moment of inertia of a sphere is 2/5*m*R^2. Depending on how accurate and what the circumstances are, you tend not to neglect things like that. Remember, assumptions make and ass out of u and me

Note that the energy imparted to the bb is from the potential energy of the spring and is then transfered via the compressed air. Now the question is the energy transfer from the compressed air change from different bb weights (this I don't know but would guess yes) while the spring energy is always a constant.

Anyways, I forget the point I'm trying to argue or the direction this thread was supposed to be taking...

[ILLusion]

Point of the thread was:

"Is it true that heavier BBs will make the opponent feel harder hits?"

And the answer, is "yes".

[mcguyver]

Is not momentum simply the kinetic energy at a given point in time?

At any rate, a BB with more mass will be less affected by gravity and air resistance and any other factors that can affect the ballistics of an airsoft BB than a lighter BB. The result is shots that are more consistant over their range, can and do have increased energy when fired and will retain their energy longer than a lighter BB. This retained energy is what causes the increased impact energy at the target.

A BB that flies all over the place may physically go farther than a heavier BB, but it's is useless for play and when it does finally strike a target, it has bled away so much of it's energy that it's impact is much less noticeable.

[Skladfin]

Originally Posted by mcguyver

At any rate, a BB with more mass will be less affected by gravity

.... =\

[Flatlander]

Originally Posted by mcguyver

Is not momentum simply the kinetic energy at a given point in time?

At any rate, a BB with more mass will be less affected by gravity.

No, and not true. Gravity affects all things equally reguardless of mass. Member the saying of dropping a bowling ball and a feather at the same time (in a vaccum) and they will hit the ground at the exact same time? It's true! Another mind blowing fact: if you shoot a bullet perfectly horizontal and drop a bullet at the exact same instant, they will BOTH hit the ground at the same time!

Wikipedia:

In general, the momentum of an object can be conceptually thought of as how difficult it is to stop the object, as determined by multiplying two factors: its mass and its velocity. As such, it is a natural consequence of Newton's first and second laws of motion. Having a lower speed or having less mass (how we measure inertia) results in having less momentum.

The kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity.

As you can see momentum and work/energy are completely different things.

A heavier BB will lose it's kinetic energy SLOWER due to it's momentum being greater.

[mcguyver]

Remember the gyroscopic effect of hop-up on a BB. The heavier it is, the more energy it has, hence the less affected by gravity. It will "appear" to the naked eye to fall to the ground slower, all the while travelling straighter.

Now this assumes you are using a decent quality gun with a decent hop-up, which most players are.

And if a BB loses it's energy slower, it's likely travelling farther and impacting the target with more energy.

That's the answer to the question posed by the thread originator.