This whole "bang-flop" thing has always been somewhat quizzical to me.
After 30 years of hunting, about a dozen years ago I started to keep a journal. I realized that I was someday probably going to like reminiscing about past beautiful days in the field and the little episodes of the hunt, and I had better start writing things down so I could remember those days and what happened. In that dozen years I have shot 30 whitetails - with shotgun slugs, muzzleloaders, handguns, and rifles. I have only lost one deer in all my hunting - a poor hit with a .44 mag handgun (not one of the 30). I lost the meager blood trail in tall grass and spent half a day more looking before giving it up.
Out of the 30, all except 3 were one-shot kills. Of that 30, I have had 4 drop in their tracks - one with a 12 gauge slug in the spine, one with a 30/30 and two shot with a .308. The rest all staggered or ran from maybe 10 yards to 80 yards and dropped dead, some maybe with a kick or two. Some ran right towards me after being shot, some ran away, some turned around and headed balls out back to the trail they came out of the woods on and piled up, some ran for the thick stuff, and some ran and staggered like drunks. One ran smack dab into a tree. Some had the far shoulder blown out and leg flopping in the breeze when they ran. I tried some of those ballistics tips in a 150 grain .308 a few years back. The boiler room looked like a soup factory, a mess, but the deer ran maybe 50 yards before piling up. I didn't use any more of those, but the last year or two I've shot a few with the Hornady SST's and I thought the wound channels looked good with a nice sized exit, so I think they hold together better.
I've shot some with their head down feeding, some walking, a few on the run. I really cannot confirm, from my experience anyway, that relaxed, or between breaths, that they fall or run because of their state of relaxation or excitement. I haven't seen the correlation that others speak of. It's always seemed pretty straightforward to me - a deer can have his heart blown out and often he still has enough oxygen somewhere to dash 50 yards (or more) and pileup. I've shot them through the lungs and had them go right down. I don't know why, but they sure don't seem to me to all act the same. Admittedly I am not that quizzical sometimes and don't do autopsies like some of my friends seem to be fanatical about. I want to get it gutted, get it hung, get it skinned, get it cool, and get a beer. Drop in its tracks, run 10 yards or 80, I figure I've done a good job and I have venison. If I could make them fall where I wanted I'd have them go down closer to my truck.
I will say though, that I have a good friend who lives in the woods in Kentucky and is an excellent hunter and marksman. He loves his .25-06 because he feels that it will regularly produce the "bang-flops" for him. His goal, when he shoots a deer, is to drop it where it stands. He tells me that with his .25-06 he kills them that way most of the time, more than with any other cartridge. I trust this fellow's word. I've hunted other game with him and shot with him and he's not a phony. It's probably thirty years since I read my Jack O'Connor books, but I do remember him describing the .270 as giving him similar results - or at least stating that he felt it killed quicker and more dramatically then other cartridges.
I liked reading Jack O'Connor and thought he pretty much told it straight.
So - "bang-flops"? All I know is that I've never had one with an elk, and in the past dozen years my whitetail average is 13%.
BTW, the one I "spined" with the 12 gauge - "bang-flopped" me too. I was still hunting and saw his legs under some low pine boughs. I bent over at the waist, leaned way forward, got him in my scope in this goofy position I was in, and shot him. So intent on the deer and the shot I didn't put that Mossberg to my shoulder. The 4X40 bushnell kissed me pretty hard along side my nose and laid it open. Had to pack sme snow on on my beak to stop the bleeding. 6 months later I had a toothache and the dentist asked me who I got in a fight with after he x-rayed the sheared roots he had to dig out. I told him I got in a fight with a whitetail, a Mossberg, and a Bushnell.
That's all I know about "bang-flops".
