Need help with ammo math

Need help with ammo math

This is a discussion on Need help with ammo math within the General Firearm Discussion forums, part of the Related Topics category; I've been trying to figure out how to calculate muzzle energy on a particular round. I would assume when we refer to energy we are ...

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  1. #1
    Senior Member Array adric22's Avatar
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    Need help with ammo math

    I've been trying to figure out how to calculate muzzle energy on a particular round.

    I would assume when we refer to energy we are referring to momentum (velocity * mass).

    So.. that being the case, most bullet weights are measured in grains. From what I gather there is 7,000 grains per pound.

    So I would think I could calculate grains to pounds by taking the grain and dividing it by 7000. So a 35 grain bullet would be .05 pounds. Does that sound correct?

    So then I should be able to get feet-lbs by multiplying .05 by velocity. Unfortunately, this math doesn't work. Every time I try this, I'm way off from the manufacturer's listed energy rating. Not even close.

    What am I doing wrong?
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  2. #2
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    You're using the formula for momentum (mv).
    You want the formula for kinetic energy (mv^2).

    Matt
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  3. #3
    Distinguished Member Array claude clay's Avatar
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    m x v2 / 452000 = energy [452000]is rounded off]

    so 125 gr going 1100 fps is 125 x 1210000 /452000 =334.62 foot pounds of energy

    if the mass is already expressed in grains, there is nothing to convert.
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  4. #4
    Senior Member Array adric22's Avatar
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    Ok.. I think I understand now. What I was trying to figure out was exactly how much difference there is in energy from a pellet gun vs. a real firearm. I'm using a standard velocity .22LR as a comparison. Typical energy is 102 ft-lbs.

    So, if I have it calculated correctly, a BB pistol using a 7 grain pellet at 450 ft-sec, should be about 3.13, meaning a .22LR is about 32.5 times more powerful.

    and a BB rifle shooting at around 1,000 ft-sec puts the energy around 15.5 ft-lbs. So that means a .22LR is still 6.5 times more powerful.

    I haven't been able to figure out the grain of an airsoft or paintball gun quite yet.. Those are usually measured in grams. But I just thought it would be interesting to make a chart showing the comparative power between airsoft, paintball, BB, pellet, and .22LR. Then maybe add in some larger calibers of real firearms too.
    "Good people do not need laws to tell them to act responsibly, while bad people will find a way around the laws." -Plato

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