Reply #112: Observations...

[benEzra]

I find three problems with your remarks.

1. You are trying to judge the size of the projectile from a video. Since you lack perspective clues such as knowing the relative size of the hand holding the round, that is a very difficult thing to do accurately.

No, it is quite clear that the round is a handgun round from the proportions of the expanded round. Expanded rifle bullets look quite different than expanded handgun bullets. Lacking an absolute size reference, the caliber of the handgun round (.357, 9mm, .40, .45) cannot be determined, but rifle vs. handgun can indeed be determined without reference to an absolute size scale. Observe:

Expanded rifle bullet, high impact velocity (which would have gone right through that vest and out the back):



A rifle bullet downloaded to handgun velocities (so as not to penetrate the vest) would have expanded less, and would have looked much like this:



Note that the rifle bullets are all quite long for their width.

Here is a pair of expanded handgun bullets; the one on the right has undergone a core-jacket separation and looks just like the one the captain is holding in the photo:



Note the pronounced cavity still present in the expanded bullet, as contrasted with the rounded front of an expanded rifle bullet.

2. Firearms are designed to mitigate the kick. Soft vests are not designed so much to reduce the impact trauma as they are designed to prevent penetration. When you stop that bullet in a short distance (thickness of the vest) your transfer of kinetic energy is much different than when the round penetrates. That is why air bags in cars work.

The "thump" is not kinetic energy, it is momentum. The kinetic energy is expended in stretching and breaking of the Kevlar fibers of the vest and is ultimately dissipated as heat; the momentum is what is transferred to the wearer. If you have excessive backface deformation, you can have injury from the deformation, but it would still be a "thump."

It is physically impossible for a firearm to transfer more momentum to the target than to the person shooting, unless the rifle has a VERY high gas to projectile mass ratio and is fitted with an excellent brake (allowing some of the forward projectile momentum to be offset by rearward gas momentum). Otherwise, forward momentum (bullet + gas) = recoil momentum (gun), period.

3. On the link above there is a diagram of bullet deformation related to speed on impact. Look at it and look at the round in the picture.

I am quite familiar with that; look at the actual photos I posted, though. That round is a handgun bullet. A rifle bullet would not have expanded significantly at any velocity that the vest in question would stop.

The "thump" comes from momentum, and cannot, based on simple physics, transfer more momentum than the gun transferred to the shooter at the moment the shot was fired. Momentum is absolutely conserved, and the total momentum (forward + recoil) of the gun-bullet-gas system must be exactly the same an instant after firing as it was before firing. Hence the forward momentum and the rearward momentum are symmetrical, same amplitude but opposite signs. Kinetic energy, on the other hand, is not symmetrical, and much more is delivered to the target than the shooter.