Ray,
Most medium to big game hunting bullets are NOT "designed to penetrate a few inches before they expand." Bergers do, but they are far from most bullets, and in fact are an rare exception.
Instead this trait was accidentally discovered when some hunters started using what were then called Berger Match Grade VLD bullets. The reason it happens is the so-called hollow-point is actually closed so tightly you can't even push the tip of a safety pin into the hole. But behind the tip there's an air-space, because the lead core ends well behind the tip, and after the bullet penetrates a couple of inches the thin-jacketed tip collapses, and the bullet expands violently. Some other target-type "hollow-point" bullets also tend act this way when shooting game, for the same reason, but they also were NOT designed that way.
All these bullets can "over-expand" if they hit something very substantial at close range, but it's not nearly as common as many believe. I was among the writers who thoroughly field-tested Berger Match Grade bullets when Berger started thinking about marketing them as hunting bullets over a decade ago.
One of the field tests involved shooting a bunch of feral goats in New Zealand, something that's regularly done over there to reduce their numbers, because there aren't any natural predators in NZ. After killing one particularly big billy goat weighing around 200 pounds, I deliberately shot the big shoulder joint from a few feet away to see if the bullet (a 168-grain .30 at about 2900 fps) would penetrate the bone. It did, and did NOT expand until after it went through the bone. There were several other instances of the same sort of performance.
But Bergers (and other closed hollow-point match bullets) are exceptions. Expanding hunting bullets with actual hollow-points, soft-points or plastic tips do indeed start to expand as soon as they hit skin--and those are vast majority of expanding bullets.
As noted in my earlier post, this has been proven over and over again, in both media and game. The difference between "varmint" and "big game" bullets is not in when they START to expand, but how much of the bullet fragments. Varmint bullets are designed to totally fragment, the reason for their shallow penetration, though a few do have a heavier jacket-base, such as Nosler Varmint Ballistic Tips, that penetrates somewhat deeper, useful on larger varmints.
Lead-cored big game bullets all fragment to some degree as well, the reason "cup-and-core" bullets with relatively thin jackets can totally disintegrate if they encounter enough resistance, say the big shoulder joint of a bull elk, or similar-sized animal.
But other lead-cored bullets only partially disintegrate. Hornady Interlocks and Nosler Hunting Ballistic Tips typically lose about 50% of their weight, due to the front end disintegrating but the rear end holding together, due to Hornady's Interlock ring, the the Ballistic Tip's heavy jacket base. Nosler Partitions usually lose 20-40% of their weight, exactly how much depending on the design of the specific Partition and what it encounters.
But the front end of all three of those bullet will expand violently on small varmints like ground squirrels and prairie dogs that are only 1-2 inches thick, because they do start to expand immediately on hitting skin. I know this from shooting a bunch of those small varmints with such bullets in calibers up to .375, and seeing the results. In fact Nosler Partitions from can explode prairie dogs very much like "varmint" bullets, because their front core is relatively soft lead alloy, to insure expansion.
But even lead-cored bullets designed to retain 90% of their weight also start expanding as soon they hit, as do bullets designed to usually retain all their weight, like Barnes TSX's. The difference between them and varmint bullets is, again, not when they start to expand, but the lack of fragmentation. This is exactly why the marmot you shot with your .338 Fail Safe didn't fly apart, and only had small entrance and exit holes. But I would bet serious money that if you'd opened up the marmot, you'd have found the bullet did considerable interior damage, because it had indeed expanded.
Barnes TSX's, Nosler E-Tips, Hornady GMX's and yes, Fail Safes, will readily expand on lighter big game. They don't do as much interior damage as lead-cored softpoints, or damage as much meat, but they definitely expand, often at extended ranges. In fact I prefer such bullets for hunting pronghorns, because there's only about 40 pounds of meat even on a big buck, and I want to ruin as little as possible. Over the years I've shot a pile of pronghorns with TSX's and similar bullets, with fine expansion out to 400 yards, as far as I've ever shot them with such bullets.
Now, occasionally TSX's don't expand, but it's unusual, and normally occurs only with smaller-caliber, hollow-point bullets from 6mm to .30 in cartridges that recoil somewhat, probably due to the small-hollow-point being battered shut on the front end of the magazine box during recoil, essentially turning them into solids. When it happens, they fail to expand at any range, even very close up, one reason my wife and I switched to Tipped TSX's and the similar Nosler E-Tip years ago.
It doesn't happen, however, with .224-caliber hollow-point monolithics. Or at least I haven't seen it happen, and neither have any friends who've used cartridges like the .223 or .22-250 Remington on pronghorns and deer. Which is another indication that magazine-battering can close the hollow-points: Neither the .223 or .22-250 recoils even as much as a .243 Winchester. I've also never seen or heard of a TSX above .30 caliber fail to open, probably because the hollow-point is much larger, so is impossible to batter closed.
Thank you Mule Deer for the thorough explanation about bullet impact and expansion.
