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Buffalobob
11-29-2011, 16:03
The performance of the RVN M21 308 sniper round with the 173 grain FMJ BT on humans beyond the transonic zone is known and documented and in my case it was even witnessed. I never personally examined the wound channels as I had no interest in cutting open NVA soldiers and the whole question was of no concern to me being as the end result was satisfactory to me. The question of the performance of the new Berger 175 grain HPBT OTM“tactical” bullet which supposedly survives the transition from super-sonic to subsonic with acceptable accuracy has become interesting to me just as a matter of nostalgia and personal intellectual curiosity.

After testing the bullets for accuracy at 100 yds and finding it to be acceptable (see other thread), I loaded up 25 rounds and waited for deer season to begin in Maryland. On Monday, a large bodied buck appeared on the hill side at 275 yards. The rifle used was a Remington 40X with an 11.25 twist 28 inch barrel, a 5.5X22X-56mm Night Force scope and a tall Harris bipod ¾ extended and dial ups were made using Exbal and a 20 degree cosine angle. A single round was fired at the deer while it was stopped in a small opening between two trees. Impact velocity is calculated to be in excess of 2100 fps. Weight of the deer is unknown but is one of the largest I have killed in the area in the last 15 years and the bone structure was fully developed.

The bullet struck the right side shoulder at the ball joint between the upper leg bone and shoulder blade bone. The bullet shattered that bone and continued its upward angle (firing angle was at least 20 degrees up) striking the front of the sternum at the last rib connection. It then hit the bottom side of a neck vertebrae and bone fragments were found another two or three inches through the neck muscles. A sizeable mass remained of the bullet and it punched and irregular shaped hole through the thick neck hide and exited. Having a degree in physics and another in engineering, I have often made calculations of momentum and impulse for bullets and elk and it is clear that in order for a bullet to exit a thick hide there must be a significant mass left of the bullet and a significant velocity or else the hide will stretch and catch the bullet.

While I am not an expert on the specifics of “tactical” bullets (whatever they are), a combat sniper bullet must be capable of penetration through some amount of hard surface such as a pack or web gear before encountering soft tissue. Berger bullets are believed by most people including the company, to be thin jacketed and to disintegrate quickly; however, this has not been my experience when engaging targets with high sectional density bullets at long range. As I have described, this 175 OTM hit three bones with soft tissue in between them and still retained enough weight and velocity to exit through three inches of neck muscle and neck skin.

Pictures are as follows:
1. The rifle and the deer with the entrance wound showing.
2. The skinned shoulder at the entrance wound
3. The rib cage with the shoulder removed and the damage to the bones of the sternum visible
4. The wound channel showing the impact on the bottom of the neck vertebrae
5. The exit wound in the neck muscle and skin.

Troop
12-01-2011, 21:23
Nice buck and an interesting story. I would not have thought an OTM bullet, even a 175 from a .308, capable of that kind of performance at 275 yards. I hope to become that kind of rifleman one day. :lifter

Penn
12-02-2011, 06:12
Yep, just checked my schedule, I do have time to drive south and pick up dinner.

Brush Okie
12-02-2011, 09:57
Great write up.

Any idea what velocity the bullet is designed to open up at? It looks like it did a good job.

Buffalobob
12-02-2011, 18:58
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.

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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. :D

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.

Buffalobob
12-03-2011, 16:17
To digress over to the shooting and hunting part of the dead deer for a minute.

I have hunted this specific area in the Pawpaw Bends of the Potomac for about 30 years now. I set up in an old field as can be seen in the picture of Phil. You can also see the tall Harris bipod extended. I know where the does have their fawns and the deer trails along the hillside and where the bucks like to bed down. I normally bowhunt this area before rifle season.

The second picture was taken about 5 or 6 years ago when there was snow cover and it shows the hardwood tree cover on the slope that must be shot through. The closest part of the hillside over the screen of trees is about 250 yards and the top of the hill is about 400 yards.

The last photo was taken after I shot the deer this year and the dead deer is on the hillside in the picture.

As you can see an accurate rifle to thread a bullet through the limbs is necessary and some good spotting optics in order to ever see the deer in the first place is necessary.