Apparently stating that bullet construction makes the most difference in penetration was too simple for such highly technical Campfire discussions.
I killed my first animal with a .308 in 1967, and my first with a .270 in 1972. Since then both my wife Eileen and I have taken over 100 animals with them, and I have also witnessed a bunch of other animals taken on various cull hunts. The animals we've taken have mostly been "deer-sized," as they are for most hunters, but have ranged up to 800-900 pounds, both in North America and Africa.
The bullets used have ranged from the simplest cup-and-cores, with relatively thin jackets and no other means of retaining weight, to various monolithics. Here's a list, which may not be complete but does cover everything from simple C&C's to monolithics:
Barnes TSX, both Tipped and hollow-point
Federal "blue box" (which used to be "red box")
Hornady Spire Point
Hornady SST
Nosler AccuBond
Nosler Ballistic Tip
Nosler E-Tip
Nosler Partition
Remington Core-Lokt
Remington Core-Lokt Ultra Bonded
Sierra Game King
Speer Grand Slam
Speer Hot-Cor
Trophy Bonded Tipped
Winchester/Combined Technology Fail Safe
Winchester Power Point
Not all of those were used in both cartridges, though the overlap is around 90%. The .270 bullets have ranged from 130-150 grains, and the .308's from 130-165, though the vast majority have been 150's. Eileen has never used a bullet heavier than 150 grains in the .308, and I've only used a couple 165's.
Here it should be noted that all "cup and core" bullets are not the same anymore, though they pretty much used to be. Many have some sort of construction feature that causes them to retain more weight than the simple cup-and-core described above. Since around 1980 Hornady Spire Points now have Interlock rings around the inside of the jacket, toward the base, and Nosler Ballistic Tips can have jackets of varying thickness. Some Ballistic Tips, in fact, are around 3/4 jacket, which means they're essentially monolithics with a little lead in the front end. I have recovered some Ballistic Tips that lost their core, but still retained over 60% of their original weight.
Over the years we have recovered twice as many .270 as .308 bullets, partly because I used the .270 with simple cup-and-cores for quite a few years before marrying Eileen, primarily the 130-grain Sierra GameKing and pre-Interlock Hornady Spire Point. Did not recover nearly as many of either as some people might guess, but one was a 130 Sierra that left its jacket at the entrance hole on a quartering-away mule deer, yet the core kept going, ending up in the opposite shoulder, killing the buck pretty much instantly. (I still have both parts.) Another was a 150 Spire Point that went through one shoulder-blade and the spine of a whitetail buck, ending up in the far scapula.
Eileen started using the .270 as her primary big game cartridge shortly after we were married, at one point taking 10 big game animals in a row with one shot, ranging from pronghorns and deer to elk to a Shiras bull moose. That was with her first .270, a Browning A-Bolt from the first year the A-Bolt appeared. I worked up two handloads that shot to the same place, one with 130-grain Hornady Spire Points (by then Interlock) and 150 Nosler Partitions. She sighted-in, practiced and shot game up through mule deer with the Interlocks, and Partitions on larger game, primarily elk but also including the moose. Then she got a NULA .270, and used 130-grain Nosler Partitions for everything.
Since 2007, however, her "big" rifle has been a custom Serengeti (now Kilimanjaro) .308 Winchester on the Kimber 84 action, which she has used both here and in Africa. She recovered three 150-grain Nosler E-Tips during its first hunting trip (which happened to be in South Africa), one from a BIG Burchell's zebra, one from a BIG warthog shot through both shoulders, and a third from a bushbuck about the size of an average whitetail. The bushbuck was standing in dense shadow, and Eileen didn't spot the Spanish prickly pear that the bullet penetrated first. The bullet has already expanded before it entered the front of the bushbuck's chest as it stood facing her, but still penetrated beyond the diaphragm.
Dunno how many .270's I've owned and hunted with, but right now only have one, a Jack O'Connor Commemorative Model 70 Featherweight. Have three .308's, an original Model 70 Featherweight made in 1953 (second year), a Franchi Momentum, and a Merkel single-shot. Have not hunted with the pre-'64 yet (may this fall), but have with both the Merkel (acquired in 2004) and the Franchi.
One of the more recent demonstrations of how even 150-grain cup-and-cores will penetrate from the .308 took place late in 2017, when I went on a group hunt in Texas with several other writers and shooting industry people. We all used Franchi .308's and Fiocchi factory ammo loaded with 150-grain Hornady SST's, a bullet I hadn't used much because of erratic results when it first appeared. All together we shot 30 whitetails and feral pigs, ranging up to big trophy bucks and boars--and never recovered a bullet. The biggest buck was shot as it almost faced the hunter, yet the bullet exited the opposite flank. The biggest boar was shot through both shoulders. Apparently the SST's have been beefed up (probably through a harder core), because even with Spire Point Interlocks I would have expected a least a couple to stay inside--especially since the shots were all pretty close, at most around 150 yards, which is where impact velocity is hardest on expanding bullets.
I do know a couple of South African professional cullers who use 180 cup-and-cores in their .308's, because component bullets are really expensive over there. The Sierras they use, in fact, cost just about as Nosler Partitions here, and they shoot more big game in a year than many American hunters shoot in a lifetime. But that is an economic decision, because they sell the meat and bullets are part of their business expenses.
There's no economic reason for most American hunters to use basic cup-and-core bullets on game larger than deer, whether from the .270 or .308r--especially when "enhanced" cup-and-cores like the Hornadys or Nosler Ballistic Tips are easily available. Have used both in the .308 and have yet to recover one from deer-sized game.
The last two .308 bullets we've recovered were a 150 AccuBond from a whitetail I shot with my Merkel, which was standing almost directly facing me at 75 yards. It broke the left shoulder and ended up under the hide of the left ham. The other was a 130-grain Barnes TTSX Eileen used on the biggest cow elk either of us have ever killed. The range was around 250 yards, and the cow was quartering toward her. At the shot the elk stumbled 20-25 yards, obviously done for, before falling. The bullet broke the near shoulder, and was recovered under the hide over the ribs on the opposite side.
From all this I have concluded that for all practical purposes the .308 penetrates more than sufficiently for general big game hunting with bullets weighing less than 180 grains.
