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Ok, so all the main topics have been touched on, but I feel some are missing.....
First, over stabilization issues are less so therefore you should error on the side of overstabilization. Under-stable bullets will keyhole everytime. Being way over stabilized like 40 gr FB in a 1:7 ar will likely just add .25-.5" to your group size...under stable like 77's in a 1:10" will likely keyhole at 25 yards and be off paper at 100!
Second is modern history. At some point we went to 1:9 for 62's....then to 1:7 for 75gr mag length rounds. I think the 75 gr ammo has not seen wide spread use.
Third, 1:9 - 1:7 will all do the most popular AR ammo with 55 and 62 bullets well enough. I load 68's mag length in my 1:9, but will not reach to 75's...no need. For 0-600 yds, the 68 bc is fine for me.
So for mag length surplus ammo, 1:9 is likely your best bet. If you see 75's or 90's in your future, adjust accordingly.
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