Lead Bullet or Rubber Bullet?

[Muhammad Danish]

Which bullet of same momentum is more effective in knocking a bear down? Lead bullet or rubber bullet?

 

[phinds]

Which bullet of same momentum is more effective in knocking a bear down? Lead bullet or rubber bullet?

What do you think, and why?

 

[Muhammad Danish]

What do you think, and why?

Lead bullet, because it will not compress on hitting the bear whereas rubber bullet will compress..

 

[phinds]

Lead bullet, because it will not compress on hitting the bear whereas rubber bullet will compress..

I think it's a matter of the rubber bullet not only compressing but rebounding slightly whereas the lead bullet will put all of its momentum into the bear.

 

[Muhammad Danish]

These explanations confuse me:

 

[anorlunda]

bullet of same momentum

You phrased the question correctly. But you should be aware that a rubber bullet the same size as a lead bullet has less mass, thus less momentum at the same speed. Also, rubber bullets usually have smaller powder charges than lead bullets.

So if your question is purely theoretical about momentum conservation, you're correct. If your question is practical, about real bullets, then there are more factors.

 

[russ_watters]

Careful here guys; if this is theoretical and the mass is the same and the rubber bullet bounces, then its final momentum is negative, not positive and the momentum transfer therefore is...

 

[jbriggs444]

Careful here guys; if this is theoretical and the mass is the same and the rubber bullet bounces, then its final momentum is negative, not positive and the momentum transfer therefore is...

The explanation in post #5 suggests that the sign of the result is irrelevant. It is the magnitude of the momentum transfer which matters. That explanation plays pretty fast and loose with signs, but as long as you plug speeds in where velocities are called for, it gets the magnitude of the result right.

 

[phinds]

Careful here guys; if this is theoretical and the mass is the same and the rubber bullet bounces, then its final momentum is negative, not positive and the momentum transfer therefore is...

Would it not be necessary to take into account the fact that the rubber bullet deforms the bears outer layers, which spring back, but the bullet drives a shock deeper into the body and there's little to no bounce-back. I realize I might not be thinking this through properly, so this is not a rhetorical question.

 

[jbriggs444]

Would it not be necessary to take into account the fact that the rubber bullet deforms the bears outer layers, which spring back, but the bullet drives a shock deeper into the body and there's little to no bounce-back. I realize I might not be thinking this through properly, so this is not a rhetorical question.

It appears that the intended result is that the lead bullet passes through the bear while the (larger, more massive, less dense, lower velocity, lower energy but same momentum) rubber bullet fails to penetrate and ultimately bounces back.

 

[phinds]

It appears that the intended result is that the lead bullet passes through the bear while the (larger, more massive, less dense, lower velocity, lower energy but same momentum) rubber bullet fails to penetrate and ultimately bounces back.

Oh, well under that assumption I don't see how there can be any question that the rubber bullet has more body impact, but that's not the assumption I would make. Bear are big damn things.

 

[jbriggs444]

Oh, well under that assumption I don't see how there can be any question that the rubber bullet has more body impact, but that's not the assumption I would make. Bear are big damn things.

Well yes. The momentum of the projectile is not the thing that will successfully stop a charging bear.

 

[russ_watters]

Actually, I hadn't read the embedded photo explanation, but it appears I was pretty close. I had the real bullet embedding, but embed or pass through, it still imparts less momentum than an equal mass and speed rubber bullet that bounces back.

 

[sophiecentaur]

Isn’t it true that the shock damage is greater from a high velocity low mass bullet?
Not sure exactly constitutes ‘stopping’.
(Just to throw in another idea.)

 

[CWatters]

I suspect they meant to add that both lead and rubber bullet had same mass an velocity.

 

[jrmichler]

Let us assume that the lead bullet has a typical lead bullet weight of 150 to 300 grains (7000 grains = 1 pound), and further assume that it is traveling at typical lead bullet speed of 2000 to 3000 feet per second. The rubber bullet has the same momentum, so it can be of similar weight traveling at similar speed, or it can be much heavier and traveling at lower speed.

If the rubber bullet has the same weight and speed as the lead bullet, it will penetrate the bear and not bounce back.

If the rubber bullet is much heavier than the lead bullet, it's time for a calculation. Assume a 250 grain bullet at 2500 feet per second, a combination that is about right for bear hunting. The momentum is 2.77 lb-sec. A 10 pound rubber bullet with the same momentum would be traveling 9 feet per second, or 6 miles per hour. It would bounce off, and the bear would probably not even notice.

Neither bullet will "knock the bear down", although the lead bullet is certainly capable of killing and dropping the bear.

And hunting bullets are made from a copper alloy, typically with a lead core.