Re: How far out will this bullet/rifle perform?
When I say destabilized I am speaking of the profile during penetration. Obviously, if you can hit it at 2100 yds, the bullet is aerodynamically stable.
What is interesting is that the same forces that make a bullet stable in flight are the same forces that make it stable or unstable during penetration of tissue. In tissue the penetrated media is a lot denser. A long ogive bullet, like the MK, will immediately achieve an overturning moment that will force it to begin swapping ends, unless it has a characteristic that immediately forces a flattening or blunting of the front end. With a MK at impact velocities above 1500 fps the forces are sufficient to cave in the HP nose forcing the blunting and mushroom initiation. At impact velocities below 1500 fps. it is not usually sufficient for this to happen before the destabilization occurs.
With your 338-416 Imp. and that 300 gr. MK you are getting mushrooming at 1000 yds and probably 1500, but I am fairly confident that that 2100 yd wound channel you describe is a destabilized bullet, or what you call tumbling. A mushrooming bullet of .338 cal. traveling at about 1500 fps. will not cause that kind of damage. Even a 50 cal won't, unless it is destabilized. Would the .338 MK penetrate both shoulders while going end over end? You bet. I would be surprised if it didn't. Actually it does not destabilize until it has penetrated the first shoulder and traveled about four to eight inches. The large cavitation characteristic of the destabilization occurs for the next six to 10 inches. Four plus six or eight plus ten gives a variable cavitation parameter of 10 to eighteen inches, which is sufficient to blow a large exit hole in the shoulder of your average elk. A mushrooming bullet would have a one caliber entrance hole and a one and one half to two caliber exit hole at those velocities. That is why I am pretty sure it wasn't a mushroom exit. Complete autopsies and postmortum wound analysis would be mandatory for certitude, so yes, that is my opinion.
Didn't you ask yourself why the exit hole seemed larger on the 2100 yd. kill than it did on the 1700 and 1800 yd kills? I don't know all of your parameters but a quick trajectory plot shows that your bullet was traveling about 1500 fps on impact at 2100 yds.
I do not enjoy pissing contests and I have too much respect for you to get into or continue one. You implied that I did not have the experience to know what I was talking about, but in this case, I do. It is an area that I have spent a lot of time and energy researching.