What is bullet momentum?

[KeithH]

What is bullet momentum as it applies to bullet weight? As in a very fast ultralight .357 Magnum round compared to a 230g +P .45 acp round with both of them generating 500 ft lbs. of energy. People will say the .45 round has more momentum. What exactly does that mean and how is it measured? (Assuming both bullets have similar construction and will not deform on impact). Buffalo Bore's website uses the example of a 50 grain .22-250 round zipping along at 3850 fps compared to a low-pressure 405 grain 45-70 round at 1350 fps with both developing a little over 1600 ft lbs of energy. That's a good picture and quite obvious but if the 22-250 bullet had an iron core and would not deform on impact why would the 45-70 have greater momentum and just exactly what does that mean?

I agree with their example but I can't define why I do nor can I get a solid explanation as to the why of the physics involved. If someone could steer me to a credible article explaining this I would greatly appreciate it.

[SCCY Marshal]

Muzzle energy features a square of the velocity and divides by a constant where momentum uses it straight:

E = (M * V²) / 450400

P = M * V

So momentum does not favor velocity like muzzle energy and the bullet bullet weight matters "more". This becomes a thing when you also look at the sectional density of the bullet, where heavier solids will have a better SD and tend to penetrate more deeply, all else being equal.

Using your 22-250 vs. 45-70 example, an iron .22 bullet would be light for caliber and have a very low sectional density. The .45 would have a healthier one with a lead or at least lead cored bullet. Muzzle energy would imply that they are the same. Cross-referencing muzzle energy and sectional densities would quickly tell you which you would rather point at a bison or brown bear. Barring extensive field experience and other people to bug about such things in field use (for better or worse), the three combine to a paint a rough picture on paper when trying to make a decision for a specific use case. It's very crude and completely disregards bullet type and construction which can throw all of that napkin math out the window. Even just a round nose versus flat point can be very different on game as small as jackrabbit while every number on paper is identical.

[the Schwartz]

As SCCY Marshal pointed out, momentum (p) is nothing more than the mass (m) of an object multiplied by its velocity (v) and is the first derivative of kinetic energy—a quantity that was first proposed by Gaspard-Gustave de Coriolis in 1829 in his paper Du Calcul de l'effet des machines* that was co-authored by Jean Victor Poncelet. Integrating momentum (mv) with respect to velocity (Δv) gives us the equation for kinetic energy, ½mv².

By themselves, neither kinetic energy (E_K) nor momentum (p) offer anything of value in the consideration of terminal ballistic performance without the inclusion of other important factors such as projectile expansion, weight retention, density and strength of the bullet and target materials, etc.




*Calculating the effect of machines

[John Hearne]

By themselves, neither kinetic energy (E_K) nor momentum (p) offer anything of value in the consideration of terminal ballistic performance without the inclusion of other important factors such as projectile expansion, weight retention, density and strength of the bullet and target materials, etc.

I'm old school but I think that momentum deserves some consideration - we can argue how much it matters - but I think its a factor. AFAIK the bullet with the most momentum will deflect less of it's original path as it encounters obstacles. It's fairly inconsequential when hitting soft tissue like a gut shot. If you're driving bullets through bones, then the bullet with the most momentum will deflect less. If you do your job and launch a projectile along an ideal path, the bullet with more momentum will tend to deflect less along that line. If I were to add any other factor, cross sectional density would be next.

[the Schwartz]

I'm old school but I think that momentum deserves some consideration - we can argue how much it matters - but I think its a factor. AFAIK the bullet with the most momentum will deflect less of it's original path as it encounters obstacles. It's fairly inconsequential when hitting soft tissue like a gut shot. If you're driving bullets through bones, then the bullet with the most momentum will deflect less. If you do your job and launch a projectile along an ideal path, the bullet with more momentum will tend to deflect less along that line. If I were to add any other factor, cross sectional density would be next.

Well, of course it (momentum) does mean something, John—we agree!

Momentum is simply one part of a much, much larger picture. Considered independently of one another each piece of the picture (momentum, energy, expansion diameter, retained mass, impact velocity, drag coefficient, density of the bullet, density of the target, yield strength of the bullet alloy, yield strength of the target, etc.) can only provide a small part of the picture.

[Shawn Dodson]

Somewhat related... many people don't know that the temporary cavity is a function of momentum transfer, not energy transfer.

[03RN]

I don't think momentum kills, or helps killing. It does help the bullet get to the parts we need to get to though.

[Noah]

I don't think momentum kills, or helps killing. It does help the bullet get to the parts we need to get to though.

Fair to say it's a means to an end?

[Unobtanium]

Tsc only tears tissue above certain velocities. The more mlmentum, the further into the target the permanent effects of TSC can manifest, provided impact velocity is sufficient.