This largely depends on who's making the monolithic projectiles. Most manufactures (ie Barnes, Hammer, and Lehigh) use a harder alloy. That's why the Hammer and Lehigh bullets fragment, and why most Barnes projectiles need at least 2000 fps on impact to expand. Cavity Back and Maker both use a softer alloy that allows their projectiles to expand down as low as 1,300 fps depending on the design. I've gone 700 yards with my Grendel and the 105gr MKZ, but it was on a prairie dog so there was no post mortem examination. The buck that I took at 300 yds had significant tissue disruption for the first 14" to 16", after which the expanded bullet continued to penetrate but didn't generate much auxiliary trauma to the surrounding tissue. Math says that the impact velocity was around 2250 so it wasn't even close to being an issue. I did shoot that bullet at 400 yds into some milk jugs (not very scientific, but its what I had readily available on short notice). The expanded bullet was lodged in the back wall of the 4th jug, and the impact velocity was around 2050. The bullet only expanded to about half its potential diameter. If I had to guess, I'd say that bullet's expansion becomes questionable below 1,800 fps, but 400 yds is as far as I can see from any of my deer stands so I haven't tested penetration or expansion past that.
What I noticed with cup and core is that they either do a LOT of damage up front, not much after and then struggle to exit OR they do a little bit of damage all the way through. Either the jacket is so thin that it expands quickly like a varmint round and looses a bunch of energy in the first 6" to 8", or the jacket is thicker and it doesn't expand a lot (not always a terrible thing), and it retains enough mass, velocity, and momentum to exit consistently. The nature of lead cores and copper jackets make it very challenging to have something in between. I used to go for the max damage. That's one reason I really liked my Savage ML. You can't fit a 45cal projectile between two ribs. If I hit a rib dead center, it would take the two neighboring ribs and turn them into shrapnel. Both lungs were normally turned to jelly with shot to the boiler room. With a 300gr, it would also destroy one or both front shoulders if they were hit. Over time, the thing that I noticed across the board was that NO ONE in our hunting group was dropping a deer in its tracks (regardless of size) unless they hit the CNS (head, neck, high shoulder). Everything else ran 50 to 100 yards and piled up dead just like they do during archery season. With archery, you can normally "eat right up to the hole" so I reasoned that so long as I'm doing a reasonable amount of damage to at least a 2" diameter path through the deer, that's as fast as I'm going to be able to make it bleed out and die. Everything else is just wasted meat. When I started testing monolithic in handguns, what I noticed was that it required a lot less energy to peal back the pedals of a soft copper bullet than what was required to make the jacket peal back and the lead flow on a standard cup and core design. With those, the impact is essentially swaging the soft core, and the copper jacket is providing resistance which causes the bullet diameter to expand more than it would if the lead could just flow back over the bullet's shank. The result was that an all copper bullet would consistently expand to some rather large diameters and still penetrate deeper than a cup and core bullet of the same weight that didn't expand nearly as much, even if it retained most of its weight. I reasoned that the only way this could happen was if the monolithic wasn't loosing as much of its velocity during the expansion process which means they require less energy to expand. As an added bonus, the expanded copper pedals are sharp so even when they slow down, they still create hemorrhaging. Looking at pics of recovered bullets from game animals, the cup and core stuff NEVER looked like the perfectly mushroomed bullets pulled out of ballistics gel. A 30 cal bullet may expand to 3/4" in gelatin or water, but on a game animal the copper jacket often folds back over on itself an the projectile only ends up expanding to .4" or so. With handgun bullets, using 4 layers of denim caused the bullets to look nearly identical to bullets recovered from game animals or in the morgue. By contrast, expanded monolithic bullets always looked very similar regardless of what they had hit. Taking all of this into account, I switched almost everything over to expanding monolithic bullets. The only exception is my ML, and that's only because I already have a large supply of 300gr SST, and (as you pointed out) it hurts to shoot that dang thing. I've spent enough time behind it on the range to know my trajectory out to 300 yds, and my personal limit on that weapon is 225 yds.
Totally agree on the accuracy issue. I don't personally feel compelled to go out and shoot hundreds, or even dozens of rounds every year like I would normally recommend to others. I spent many years under instruction sending hundreds of thousands of rounds down range with a methodical nature that bordered on mechanical. Many of those rounds were fired at national events. Shooting isn't like riding a bike for me. For me, riding a bike is like shooting a gun.... That being said, when I pick up a new cartridge to hunt with, I put it through its paces before taking it in the field. I can't think of a place here where I could take a shot further than 300 yds on a deer, but if I'm going to shoot at game farther than 150 yds, you can bet I've run it out to 500 on paper multiple times before it ever goes in the field.
