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Altaïr

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How do you calculate drag?
Drag = 1/2 × ρ × S × Cd × v^2

ρ = 1.225 kg/m^3 : air density. That's at 15°C at sea level (air density varies with temperature and altitude), though you can use this value, it will give you a good idea.

S = π×R^2, where R is the radius of your projectile (it's a sphere right?): cross sectional area

v: the projectile speed: 25 m/s for you.

Cd: drag coefficient. For a sphere in those conditions, you can assume it's 0.5.
 

Chara-cter

37°14′0″N 115°48′30″W
Drag = 1/2 × ρ × S × Cd × v^2

ρ = 1.225 kg/m^3 : air density. That's at 15°C at sea level (air density varies with temperature and altitude), though you can use this value, it will give you a good idea.

S = π×R^2, where R is the radius of your projectile (it's a sphere right?): cross sectional area

v: the projectile speed: 25 m/s for you.

Cd: drag coefficient. For a sphere in those conditions, you can assume it's 0.5.
so it's 1/2 x 1.225 x πx0.003^2 x 0.5 x 25^2= ~0.005
Did I do it right?
 

Altaïr

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Yes. The value seems very low indeed (I suppose that's what bothers you), but if it's applied to an object that weigh 1 gram (so 0.001 kilogram), you get a deceleration of 5 m/s^2, which is noticeable.
 

Altaïr

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This object weigh 0.3 gram so I don't think it would be noticeable
On the contrary, it has greater impact on a light object. The deceleration from drag is a=F/m. In this case the initial deceleration would be 18 m/s^2. It would decrease quickly though (because drag is proportional to v squared). A heavier object would have more inertia.
 

Chara-cter

37°14′0″N 115°48′30″W
On the contrary, it has greater impact on a light object. The deceleration from drag is a=F/m. In this case the initial deceleration would be 18 m/s^2. It would decrease quickly though (because drag is proportional to v squared). A heavier object would have more inertia.
hmmm
interesting
 

Mooncrasher

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This object weigh 0.3 gram so I don't think it would be noticeable
Think like this:
Imagine a constant 100 kg (which works out to approx 1000 N) push. That's enough to slowly slow down a small car. But will a semi-truck notice it?

For a given force, it will effect a lighter object more.
 

Chara-cter

37°14′0″N 115°48′30″W
On the contrary, it has greater impact on a light object. The deceleration from drag is a=F/m. In this case the initial deceleration would be 18 m/s^2. It would decrease quickly though (because drag is proportional to v squared). A heavier object would have more inertia.
So how fast should the BB fly to reach 25m? 43m/s?
 

Altaïr

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So how fast should the BB fly to reach 25m? 43m/s?
If your only concern is to reach a target at 25 meters then it's fine, it will just need more than 1 second.
Also, just because the initial deceleration is 18 m/s^2 doesn't mean that the speed will drop from 25 to 7 m/s in one second. As the speed decreases, drag decreases as well, so the deceleration decreases too. The distance it can travel in a given time can be calculated, but it's a bit tricky. I can do it for you if you need.
 

Chara-cter

37°14′0″N 115°48′30″W
If your only concern is to reach a target at 25 meters then it's fine, it will just need more than 1 second.
Also, just because the initial deceleration is 18 m/s^2 doesn't mean that the speed will drop from 25 to 7 m/s in one second. As the speed decreases, drag decreases as well, so the deceleration decreases too. The distance it can travel in a given time can be calculated, but it's a bit tricky. I can do it for you if you need.
can you calculate its ballistic? Like this one
1597396063988.png
 

Chara-cter

37°14′0″N 115°48′30″W
Anyway, this is how I'm gonna make the inside of the gun. It's kinda easy to understand how it works (I guess). If you don't understand I could build a prototype and make a stop-motion gif from it
Blue: Barrel
Orange and red: Mag parts
White: The BB (Sorry if u can't read this lol)
The other white: part of the trigger group

The other blue: Part of the firing pin (that act as a bolt carrier group)
Brown: The firing pin
Green: The old bolt carrier group (Now won't move)
Pink: trigger (the part you pull)

Grey: springs, other parts,...
Darker red: Safety selector

1597396194326.png
 

Altaïr

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can you calculate its ballistic? Like this one
View attachment 43792
I can, but you shouldn't expect it to be very reliable, as your projectile is very light, and it will be very sensitive to variations in atmospheric pressure, wind and so on. I can't guarantee that the result will be accurate.