I haven't killed a real deer with the copper bullets, and I'd be the first to agree my 'testing' is unscientific, but poor people have poor ways. Wet sawdust boxes in summer and compacted snow boxes in winter have revealed that most copper bullets are too tough for modest velocities. I have fooled with 6.5 Swede, 7x57, 7.65 Mauser and 8x57...and the early coppers acted a lot like solids. The 'control/comparison bullet was Rem CoreLokt. 150 yd impact velocities roughly 2,250fps to 2,550 fps. Unimpressive result with very small frontal area expansion. I just bought a kings ransom in Hammer Bullets of various calibers, the style is the Shock Hammer...designed for lower velocities, testing will resume when the weather lets up. If nothing else the Shock Hammers are easy peasy to work with accuracy-wise...no chasing seating depth.
Gee, Eileen and I and several companions have now taken around 100 "real" deer and other animals with "copper bullets," mostly Barnes Xs of various types, but also Hornady GMXs and Nosler E-Tips. There are also a couple of others that work the same way--the discontinued Combined Technology Fail Safe, which had a hollow-point front end that acted exactly like the original Barnes X's, and steel-capped lead core in the rear end, and the North Fork, which is essentially a hollow-point monolithic with a tiny sliver of lead in the hollow-point to enhance initial expansion.
I have used a bunch of stuff to test bullet expansion over the decades, starting in the late 1980s, including ballistic gelatin, the Test Tube which used a variety of wax, and various home-made stuff like Bob Hagel's 50/50 mixture of damp sawdust and sand, and stacks of wet newspapers and dry newspapers. Have even placed larger bones of freshly killed big game animals or domestic animals including beef cattle and elk in front or inside various kinds of media.
While I have found expanded bullets on the ground after the snow melted on a private range I used for several years, have never considered using only sawdust, as it Hagel's mix was considerably tougher.
During all this I recovered enough expanded bullets, which killed and and other animals up to 1500+ pounds, that they're stored in an over-sized tackle box. They vary from .224 bullets weighing 40 grains to .416 bullets weighing 400--and muzzle velocities and ranges varied enough that several impacted at around 2000 fps. One example would be an original Barnes X-Bullet, a 120-grain 6.5mm that grouped very well in a Ruger 77 Mark II I owned and hunted with quite a bit for several years around 2000, at a muzzle velocity a little over 2900 fps.
In 2003 I used it to take a Colorado pronghorn buck at a lasered 371 yards, with a broadside shot behind the shoulder. Back then stories about how Xs sometimes didn't expand at lower velocities were not uncommon, especially if they didn't hit substantial bone. Now, pronghorns aren't very big (the heaviest field-dressed buck we've ever weighed went 93 pounds) so there isn't much resistance to cause bullets to expand, so I was both interested and nervous about the result--but at the shot the buck stumbled forward about 10 feet and fell dead, because there was a good-sized hole through both lungs.
I still have a copy of the very first
Barnes Reloading Manual, which lists the G1 ballistic coefficient of that bullet as .441--which it may have been, given the usual method many bullet manufacturers used back then, chronographing both at the muzzle and 100 yards, or maybe 200. But BC tends to drop with velocity, which is why that method really doesn't work with today's high-BC bullets beyond about 200 yards.
Anyway, the listed BC indicates that bullet hit the pronghorn at around 2100 fps, where according to your "tests" it shouldn't have expanded. But have also used TSX bullets at muzzle velocities as low as 2200 to shoot feral pigs, and they worked fine too, at ranges out to 200+ yards.
The test I really trust most for hunting-bullet expansion is shooting "real live" animals, whether deer or anything else. And the test I eventually came up with that tended to reflect actual result from bullets recovered from game was stacks of dry newspaper, which were the closest match in both penetration and weight retention. Have used it a number of times when a new bullet appears on the market, before I hunt with the bullet. Might even post a photo of the results from a line-up of several 7mm bullets in the 160-grain weight range later, which I did for a book titled
Rifle Bullets for the Hunter, A Definitive Study, compiled, edited and partly written by my fellow writer Richard Mann, the guy who developed the Test Tube, which was published almost 20 years ago. Several other writers contributed chapters, including Craig Boddington, one reason that copies now sometimes go for over $200.
There's plenty of information out there about how expanding bullets actually work on game--and what pre-hunting tests actually work pretty well.