the best commonly available bullet in 24 caliber is probably the 105 Amax, but Berger does a 115 hunting bullet with a very high B/C. Both are plenty good enough for deer at 500 yards. What case you use is your business, but I see little difference between the 6/250 improved and the 6mm-06. A look thru loading manuals will tell you that the .243/6mm Remington capacity are about perfect. The rest simply blow powder down the bore. This even goes for the 6mm Ackley even though it's far less at wasting powder. 44 grains of H4831sc in the 6/250 improved case will get you 3000 fps. So will Retumbo and H1000.
In the 6mm Remington you can reach 3150fps with 45 grains of IMR 4831 (very max), but if you use old and original .257 cases to form 6mm brass you can get another 75 to 100fps (those cases are very rare these days) as they hold a grain and a half more water. The 6mm Improved is maybe a 150fps faster than the normal 6mm, but also uses more powder yet. The 6/284 comes right in there with these rounds, as does the 240 Weatherby. This all fits right into what Ackley often said about the 24 bore. The 6mm was about the max the bore would handle without going into over bore. It's also why the .250AI shoots better than it's supposed to be able to. The .257AI will probably do about 3150fps max with a 115 grain bullet (I mean VERY max). The 25-06 is also right there as well. The .257WBY will get you about 150fps with a minimum of 20 grains more powder (8700). This is a great example of overbore. Yet the 270mag will easily push the 130 grain bullet just as fast (same case) at 3300fps. The 6.5WSM will do about 3250fps with a 140 grain bullet, and shoot the 130 grain bullets another 100fps faster with a heavy load of Magpro. Now Nosler has an Accubond Long Range bullet that's about 130 grains with a .6+ B/C. Calculate that out! Even with an accuracy load of 3150fps (maybe), this is a serious round! I can't see that far in my old age! Of course the 6.5x57AI or AAR will put you right on 3000fps with the same bullet.
What the above shows us is that a certain bore size will only handle just so much powder. Ackley told us that forty years ago! And somebody else told him, I'm sure. A very pointed example here is to compare the little 6BR with a 105 grain bullet. You can easily get 2800fps from it (pretty stiff), and still not have case problems. Some folks are getting 2900fps, and one guy was getting 3000fps (probably a 68K psi load). Then you compare that with the big boys in that bore that are using almost double the powder! The 6BG will do 3000fps with 40 grains of powder, and yet it's about .250" longer than the 6BR, but also with lower pressures. It takes another four grains of powder in the .250 improved case to get there
gary