Most factory cases that become Ackley Improved's are not hard to form, for many years it has been said that the parent cartridge could be fired in an AI chamber without any problems. I forget the reason why, but it was once explained to me in detail by a gunsmith, and it made perfect sense. I just suffer from CRS sometimes
Some Ackleys are more difficult, because the shoulder is actually moved forward and then blown out to 40 degrees. My 6 Long Dasher is a perfect example of this, but it's not a standard AI by any means.
The guy that built it told me it would out perform the 243 AI. I don't know for sure, but I do know that I can easily get 3300 with 105's and H4350. It fits in a Ruger short action with plenty of room to spare. Probably very similar to the 6-250 AI that Tricky is talking about, and there are alot of good reports on that cartridge too.
260, 7-08, 6.5-284, 6.5X47 Lapua are also good choices IMO. Especially if you are interested in deer at 700 or 800 yds. The 6.5-284 probably holds the candle out of those 4.
In order to get the ballistics of a good 6 or 6.5, you've gotta go really heavy bullets in 30 cal. In fact, ballistically the 6 Long Dasher with 95's at 3300 has less drop and drift at 1000 yds then the 300 WSM with 180's at 3000 does. It doesn't have the energy or "knock down" power at 1000, but it's ballistically more friendly.
Lots of choices out there nowdays that are better than the 308 IMO.
here's why I said to go with the 6/250AI instead of the .243AI. When fire forming an improved case the over all length actually shrinks anywhere fron .030" to .060"; depending on the case design and how much of a void the brass has to fill. A typical .243AI shrinks anywhere from .03" to .045", and the neck is the part that looses out. The .243 neck is already too short for heavy bullets and very light bullets, and the T.P. ends up in the throat killing barrel life. But with the 40 degree shoulder, it helps by moving the T.P. back into the neck a little bit. You still can't overcome the too short of a neck syndome. Most guys I know that still shoot the .243AI use 6mm Remington brass instead of generic .243 brass to at least keep the proper neck length. My 6/250AI chamber is actually about 1.968" long, and I use .243 brass rather than 22-250 brass. My cases will shrink right at .040" on fire forming. This makes them come in at about 1.930" for a neck length of .26" (roughly). I'd much rather have a .290 neck length. My next 6mm round will be a 6BG or a 6BG+.100".
gary
I was very excited when the 6mm Dasher came onto the scene and could not wait to get one build and get the magical numbers they have been reported to produce.
In the end, it topped out at around 2900 fps with a 107 gr SMK and has done so in many rifles. Some will rarely push 3000 fps but thats pretty hot on average.
By comparision, the unsexy 6mm-250 will drive a 107 gr SMK to 2950-3050 fps depending on barrel length, shoot into the 1/5 moa range depending on rifle and shooter and no fireforming needed.
The Dasher lost a bit of luster in my eyes after testing it. Still a great round but just realized what your getting and do not expect magic. IF that is the case you will be very happy with the Dasher if you like forming brass.
Any 6mm is pretty light for true long range deer hunting. The 6.5mm is a far better choice and offers much better barrel life with very little increase in velocity.
I guess it comes back to what Long Range means to a certain individual.
Why a 6mm/250AI? your giving up 10 grains of capacity to the 243AI if both are loaded to the same length and the same quality brass is available for both. What is better, just wondering? I can push BN Dtacs nearly 3200 with 243AI and acccuracy is great.
The difference is about nine grains of powder (or less) for about 125 fps less velocity. I could push the 117 Sierras even harder, but don't. 105AMX bullets at 3100 fps is what the barrel likes. The bullets are still seated into the shoulder a bit more than I would prefer, but nothing like your 115's are. Most every .243AI case ends up being .220" or less in the length (most are .21"). I finish out at .260", and trim to .25".
gary