Bullet size/BC. Pick a .257, a 6.5, a 7mm, or a .308 caliber. All of them can kill to 1000 yards with the high BC bullets. The .257 is the lightest at about 130-135 grains, but that is definitely enough if you put it in the boiler room behind the shoulder. The difference comes in wind deflection, heavier bullets deflect less in the same wind. I won't go above 180 VLD in 7mm because it is the maximum recoil I am willing to put into my shooting system. Increasing recoil in field conditions decreases my ability to control recoil to spot shots and control the recoil while the barrel is in the bore which does affect group size. It is diminishing returns, because I would need to shoot 235 grain 308 bullets in something even bigger than a Win Mag in order to get the velocity needed to match the external ballistics of the 180 vld in 7mm. That is why a 7mm magnum shooting 180 VLD is superior IMO to typical 300 win mag and 300 PRC loads. At the 230+ grain .308 bullets with all that powder puts recoil beyond what I am comfortable with reliable and consistent shooting. (I won't shoot anything but suppressed and won't carry a heavy rifle).
Velocity. A high BC hunting bullet pushed between 2900 fps and 3100 fps is a sweet spot for loading, recoil, barrel life, ES, etc. Elevation dope is the same for bullets regardless of weight if the BC and the velocity is the same. Wind deflection is greater on the lighter bullets over the same TOF.
Short mag. There is also something about the bullet weight to powder ratio that makes it easier to load for and keep low ES (extreme velocity spread). The higher the ES, the larger the vertical spread will be at 1000 yards. It is pretty well known by a lot of shooters that short fat cartridges are easy to load for and have very consistent performance. 6BR/BRA/Dasher, 7 saum, 300 short mag, Sherman Short mags and Sherman Short Tactical, etc. I ran 6-7 grains less powder and shot 180 vlds at 2970 out of my 7 SS than shooting the same bullet at 2950 out of my 7 rem mag with more powder. The difference in powder makes them more efficient and so much easier to load for, and more consistent groups.
Wind Drift. In good wind conditions, the wind deflection of the 180 gives me a margin of error that is small enough out to 1000 for easy to call wind conditions. My 25 SS is still getting built, but the dope comes out really similar. A 6.5 PRC with high BC bullets can do the same thing, and with a little less recoil it is a fantastic choice. To get less wind drift than the 170 VLD I would need to get a heavier bullet at the same velocity, and that puts me back into the 235ish .308 bullets. I won't argue .308 can't be ballistically better, and doesn't whomp critters better, it can and it does. My argument is the recoil isn't worth it to me because any increase in precision/accuracy by a bigger bullet going faster is offset by the recoil that makes shooting it harder for the vast majority of guys. A lot of you dudes shooting RUM and saying you don't feel recoil, I am just not buying the machismo. The handful of guys running RUM that I have hunted with get worked by the recoil. I have seen guys who manage it, but they are serious dudes and have exceptional shooting form and recoil management. They also have a little more weight in them for that reason.
Shooting position. I prefer and practice shooting off my tripod. I can shoot my off my tripod in field conditions pretty much as well as prone off a bipod. Consequently, if I can sit down and get three legs of my tripod on the ground, I can shoot confidently to 1000. I actually like sitting on the slope of a mountain with my feet below me, because of the configuration of the tripod and how I build my position. That is pretty common for hunting in AZ like I do. I also don't like lying down if I could find a spot cause there is invariably cactus, lol. Standing off a tripod, I am confident to 500. But, I think with my new tripod and rifle configuration I could push it out further. If you don't practice in the field with a tripod, then stick with a bipod and rear bag for long range, with shooting sticks for intermediate distances.
Kill Zone. There are limitations on distance I will shoot depending on size of kill zone. An elk is large enough that I will shoot all the way to 1000 yards and the 1800 fps limit of my 7mm. But, a Coues deer vitals aren't big enough to absorb the known group size and the unknown confounding factors for DOPE. For instance, on the range, I can consistently shoot an 8-10 inch group with my 7 ss in no wind or limited wind situation, and first round impacts as well. (It doesn't count if you shoot a small group but miss your first or first few shots.) When factoring in the wind call on top of that, then the acceptable range reduces as the wind increases. In a 7 mph wind, my 7mm 180 vld deflects .1 mil for every hundred yards. So, out to 1000 yards, a 7 mph wind will drift it .7 mil. If I call wind at 10 mph but miss the wind call by 2 mph, then that would be about .2 mil additional error. Lets say my group size is already .2 mil at 1000 yards, add on bad wind of .2 mil and at the extremes that means I could miss left by .4 mil. At 1000 yards, a 1.0 mil target is 36 inches and .1 mil is 3.6 inches. So, the .4 mil error is 14.4" at 1000 yards. That's why elk are on the menu at 1000 yards, but Coues deer really can't be.
Don't be a hero. I can usually close distance, and I will shoot to 1000 yards only if the terrain limits me. I will always get as close as I can down to 500 yards. I usually stop and take the shot while the option presents itself if I get to 500 yards. I don't really need to get closer for complete confidence. When I can shoot a 4 inch group on a rock at 500 yards from a field position and my first impact is center of the group, I see no reason not to pull the trigger as soon as I can set up at 500. Shooting suppressed, I actually feel like I would have more opportunities at 500 if I were to make a bad shot because game just don't react to the shots. Strangely, I have more confidence at 500 yards than shooting offhand at 100 yards. That comment makes me realize I just need to practice offhand shots more, lol.
That pretty well captures what I think about long range. And, it probably confirms you all think I overanalyze stuff, which I do. All of that doesn't mean anything if I didn't do the work with my gear practicing and dialing in the gear I chose. I have my rifle doped, data trued, and range proven to 1000, I have redundant ballistic calculators, I understand everything about my technical gear, I have 100% confidence in my rangefinder (and that I accurately ranged the animal and not a tree in front or behind), I don't let my ammo get too hot or too cold in the field, I load my ammo to spec and tested with low ES, I keep my rifle system and tripod in operating condition, I only use the same quality scope and reticle that won't crap out on me, I test and confirm bolts are torqued and haven't moved on the rifle, I keep the barrel appropriately cleaned and fouled, I have a shooting checklist that is natural to build my position and operate my rifle so I don't forget to dial my dope or leave dope dialed on, and more. If you can do that or more, you too can be a long range hero.