Northeastern Oklahoma had a lot of quarter-mile section fences, especially around Chelsea. Whitetails died quickly at a quarter mile with 85 grain Sierra HPBT bullets started out at 3000 fps muzzle velocity. There is a lot of open pasture land there. The shots were taken from one fence across the open pasture to deer walking down the other fence line. As a kid, I watched my shooting mentor do this several times over the years. He normally used the heart shot with his 6mm Remington in a M700 BDL Varmint Special.
A large number of coyotes met their demise at the same distance from the same load. Crows sitting in some of the Osage Orange trees along the fence also met the same fate. That particular fence line was the game highway.
At 600 yards, the same bullet would just pencil through a coyote. My friend felt that 600 yards was about the limit for clean kills on coyotes and did not shoot deer at that range.
The same load also killed hogs in Southeastern Oklahoma on timber land at much shorter ranges.
My shooting mentor was an exceptional rifleman.
Many years later, I took large Western Kansas whitetails/mule deer with my 6mm using the same bullet at the same speed around Plainville. There is a lot of open space there. You can get close shots in a gully or creek bottom. You can also get long shots across a stubble field. And in that part of Kansas, wind is almost always a factor. I made a kill one year at over 400 paces across a wheat stubble field. That was a big doe, much bigger than those in Oklahoma. Those big round hay bales make good shooting rests.
If a 6mm Remington reliably kills deer at 440 yards, I suspect that the standard .243 Winchester would, too. I am not sure how much further the improved version will extend the range. Other factors such as wind, mirage, and shooter ability get real important much past 300 yards. These days I have a self-imposed limit of 250 yards. Years ago I burned up a lot of ammo in practice and varmint shooting. These days I am lucky to get to the range a few times per year.
In open country, I would feel confident with that setup on deer were I still up to the task. However, I lived in South Carolina for a few years and did some shooting across soybean fields from tree stands. There I would not use the .243 on deer at that kind of range. The reason is that sometimes the deer will run a ways even with a heart-lung shot. The bean fields I hunted over were surrounded by thick woods. The bullet did not always exit and sometimes there was not much of a blood trail. In Oklahoma and Kansas we could see literally for miles and could follow a deer with our eyes for 40 to 50 yards if one ran. In South Carolina, I lost a doe that ran from the field into the woods. We found it the next day with a dog, but the meat was lost. The hit was good but no blood trail. After that I switched to a heavier caliber.
So, in my experience, the 6mm will kill reliably out to a quarter mile. Also in my experience, the deer will sometimes run a few yards. I would not use it, improved or not, at that distance in wooded country.
