Updated: Jul 7, 2020
This discussion will address real experiences in the hunting field and the reasons for those. To focus on the issues that will drive the discourse: below is a table of pre-impact, raw momentum figures plus the impact impulse per sq. mm frontal area that bullets from some well-known cartridges possess. Pre-impact momentum is given in Newton.seconds which is another way to express kilogram.metre per second.
Penetration Impulse is given in Newton seconds per square mm. of bullet frontal area, which is the actual penetration force of a bullet. (BTW the bullet's retained frontal area and the pre-impact momentum can also be seen as expressing a bullet's retained sectional density during impact and penetration):
Cartridge Bullet Weight Momentum Penetration Force per sq. mm.
6.5x57 Mauser 160 gr. 7.6 N.sec 108 N/sq.mm
7x57 Mauser 175 gr. 8.3 N.sec 104 N/sq.mm
.375 H&H 300 gr. 14.3 N.sec 104 N/sq.mm
.416 Rigby 400 gr. 18 N.sec 103 N/sq.mm
30-06 Sprg 200 gr. 9.5 N.sec 103 N/sq.mm
.458 Lott 500 gr. 21.8 N.sec 102 N/sq.mm
9,3 x 62 286 gr. 12,9 N.sec 95 N/sq.mm
.458 Win Mag 500 gr. 19.3 N.sec 91 N/sq.mm
.45-70 Gov 405 gr. 10.4 N.sec 47 N/sq.mm
Comparing the momentum figures of the listed cartridges it is clear that a raw momentum figure has absolutely no bearing on a bullet's penetration impulse because momentum is not a force unless it is integrated with the time of change in momentum it is subjected to. Furthermore unless the impulse force in integrated with the bullet frontal area even impulse has no bearing on penetration.
As an example, the .45-70 has a 70% higher momentum figure than the 6.5 x 57 Mauser, but the little 6,5 mm bullet has 129% higher penetration ability than the big .45-70.
So: “Does the 6.5x57 with its its mere 7.6 Newton.sec. momentum but impressive 108 Newton.sec impulse per sq. mm frontal area penetrate the same with a frontal brain shot on elehant as the .458 Lott with 300% more momentum?"
Indeed it does exactly that. The 6.5x57 Mauser, and the slightly less powerfull 6.5x54 Mannlicher Schönauer cartridge which was used by Bell with a 159 gr FMJ bullet have repeatedly proven the penetrating ability through the bone structure of the front skull of an elephant. In fact, the whole family of old 6.5mm cartridges has been well known in Africa for longer than 120 years - the 6.5x54 MS, the 6.5x55 Swedish Mauser, the 6.5x57 Mauser, and the 6.5x58 Portuguese Mauser - for having all done that many times. These cartridges, and the 7x57 with 175 gr and the .303 Brit with 215 gr FMJ bullets have killed countless elephant with frontal brain shots.
But there is a caveat to this.
Many hunters who have observed their .264 Win Mags with 130 gr expanding bullets to penetrate impressively many inches of hard oak leaving a pencil sized hole all the way, also experienced bullet break up on an animal's shoulder and lack of penetration. A well known USA gun writer and author experienced exactly that on a hunt in Namibia a few years ago. The gemsbok, zebra, kudu, wildebeest were all wounded and only one springbok afforded sufficient penetration for the bullet to reach the heart. One must admire his honesty in reporting his wounded game - but his final contention that the .264 Win Mag is not suitable for Africa game is patently incorrect - way off the mark. For the best part of 130 years 6.5mm cartridges have been used very successfully on all Africa game. If the hunter employs lightweight bullets (he used some or other 120 gr Nosler) which by design will fragment and therefore can not and will not penetrate the shoulder bones of big game like gemsbok, the result of a wounded animal is not by virtue of the geometry of the cartridge case.
Back to penetration through hardwood: the hard monolithic structure of the oak lets the hole behind the nose of the bullet act as a sleeve which envelops the bullet shank and literally prevents the shank to wobble or tumble. The monolithic structure of the elephant front skull also does exactly that to the long, thin, high sectional density 160 gr 6.5 mm bullet. The walls of the calibre size hole through the monolithic skull structure keep the bullet's tail behind the nose so there are no side forces onto the bullet shank that will add to extra drag against penetration.
What about Cape buffalo?
Will the long, thin 160 gr bullet from a 6.5mm cartridge with its impressive 108 Newton.seconds impulse per square mm also penetrate a Cape buffalo shoulder, nose ahead through the polylithic structure of 1" thick skin, very tough and slippery tendons, break the very hard, convex humerus bone, break through 1" thick overlapping ribs, slice open the heart top chambers, break through 1" overlapping opposite ribs, break the opposite shoulder joint of the humerus-scapula bones, slice through a thick, tough, very slippery tendon and punch through the skin and be gone like a 480 gr bullet from a .458 Lott will do which has only a little higher impulse per square mm bullet frontal area?
It most certainly will not. Particularly will the very hard, convex outer structure of the humerus it suddenly encounters after having entered the shoulder tendon and muscle group bend the long bullet, causing a tumble and immediate loss of penetration. A frontal brain shot will work if placed correctly depending on the buffalo's head attitude because there is mostly a monolithic mass of bone to overcome.
There is another observed issue with Cape buffalo shots into the low shoulder with the decided object to break the humerus bones on both sides which I personally have not completely thought through yet: that is, despite a bullet's penetration figures, the apparent need for sheer mass to be able to overcome all the obstacles afforded by the polylithic structure - apart from the factors of impact momentum and retained sectional density during deceleration. In a way it has to do with the bullet's amount of matter and its ability to absorb the heat translated into it during slowdown, when kinetic energy is translated into raw heat - when latent joules due to kineses decrease and actual joules of heat energy is produced within the bullet.
In summary: in order to highlight the real limits of penetration ability the discussion centered around heavy, dangerous game. The same principles apply when thin skinned, thin boned animals are hunted. A 168 gr Peregrine Flat Nose VRG-2 monolithic solid from either my 30-06 or .303 Brit will cleanly penetrate the low shoulders of any antelope including eland, create calibre sized entry and exit holes and a 4x calibre size permanent inside wound channel with zero meat damage.
I see no need for using any expanding bullet anymore as it affords no advantage. Because we aim for the heart top chambers and not "behind the shoulder" into the lungs this makes a lot of sense. It is just a fact that lung shot animals live long and may be lost in the dense vegetation some habitats present and therefore no one shoots "behind the shoulder" into the lungs only.
It was mentioned earlier that the Peregrine Flat Nose VRG-3 expands to 1.8 x calibre under most hunting conditions of distance, impact velocity and animal size. That means that the 48 square mm frontal area of the monolithic solid 200 gr 30-06 bullet will expand to (7,82mm x 1.8) = 14mm diameter. That gives it a new frontal area of radius squared (7x7) x Phi which is 154 sq. millimeter - about 300% larger frontal area which will greatly increase the penetration-opposing drag force on the bullet.
Applying the impulse of 4,750 Newton.sec to this new expanded surface the new impulse force is down to 30.8 Newton.sec per square millimetre frontal area - from the solid bullet's impulse force of 103 Newton.sec per sq. millimeter.
This impulse on the expanded frontal area is still sufficient to penetrate the low shoulder of a wildebeest at 280 yards, break both shoulder sets and bulge against the opposite skin.