Quote:
Any idea what velocity the bullet is designed to open up at?
|
No I do not. However, I was out chasing deer around this morning with a revolver and got to thinking that maybe I should section the bullet and look at jacket thickness of the Berger. Below are my thoughts on expansion at extreme range
The old 173 FMJ BT did not open at any speed and still worked remarkably well.
The Sierra 175 MK HPBT has a relatively thick jacket but does not survive the transition zone with accuracy so there is little point to considering it at the longer ranges and whether it does or does not expand.
Depending on atmospheric conditions the transition zone occurs as the bullet approaches 1100 fps. At this time the Berger or even the old FMJ has about 485 ft lbs of energy which is equivalent to a .40 cal pistol at point blank range (bullet diameter is considerably different). So we still have enough to kill a person but not a lot to waste and this is important to not be wasting what we have got left. So at this point let us hold these thoughts and turn to what we know about physics and re-examine some of the things we learned long ago.
First thing to remember is that conservation of energy is only a teaching concept but has no application in the real world. In the real world we deal with conservation of momentum. If we lose energy we lose momentum being as both have the terms mass and velocity.
There are three kinds of collisions between two objects
Completely (perfectly) inelastic such that when one object hits another object all momentum is lost and a good example is dropping an egg onto the floor. It does not bounce and all energy and momentum are lost through deformation of the egg.
(Partially) inelastic such that when the two objects collide some momentum and energy is lost but some is retained. A good example is dropping a ball onto the floor. It will bounce a little but most of the momentum is lost and more is lost each time it bounces.
(Totally) elastic collisions where the sum total of momentum of the two object following the collision is the same as before. The little steel ball pendulum gadgets do a pretty good job of this but will eventually stop.
So we can deduce that a bullet striking an object and in this case we will say the target is a deer is a partially inelastic collision. Energy is lost through moving the tissue out of the path and friction as heat while passing through the tissue and when bone is struck there is the momentum imparted to the bone and the energy of deformation of breaking the bone and the energy of deformation of the bullet, if it expands. Here is the crux of the issue at hand. An expanding bullet will lose about half of its mass through deformation and collision and thus reduce its momentum by the 50% mass loss; while an non expanding bullet will not suffer that loss. So if a bullet strikes a deer and encounters bone the non-expanding bullet will retain 50% more momentum than the expanding bullet and be able to penetrate that amount further into the target. Two extremes may exist which lead to endless debates amongst the hunting community. The expanding bullet will deliver all or a lot of its energy into the target and the FMJ may retain a lot and exit without delivering much to the target.
So now let us return to the rifle bullet that has traveled so far that it has only as much energy /momentum as a pistol bullet. Clearly we do not have a whole lot of energy to waste so we need to make the best use of it that we can. The fact of the matter is that we probably do not have enough to break the shoulder bone of a mature elk if we are using an expanding bullet. So at ranges beyond the transonic zone with a 175 gr bullet one is back to the old problem of the pistolero and whether to load up hollow points or round noses. In the case of the military one should be prepared to have to shoot through a guys back pack and you would not wish for your bullet to expand and lose its momentum hitting his can of beanie weenies there in his pack. So if you wish to be prepared to penetrate through a certain amount of combat gear you really are not interested in a bullet that expands at low speeds.
Conclusion is that expansion at velocities less than mach is a waste and unwanted on a military target.
-----------------------------------------
In the past a few physicists and engineers have taken exception to some of my collision terminology and may do so if they wish but I have tried to indicate that I actually do know the correct words; I just like mine better.
Caveat: I am not the chief bullet procurer for the United States Army and my opinions of correct bullet performance at extreme range are based upon only a very small sample.