I'm with Metal God on this one... Look at the shelves right now and tell me what you see.
In this area, 30-06, 7mm08, 30-30, any magnum, 7x57, 6.5x55, are all available around here. Just walking through BPS about a month ago, as long as you WERE NOT looking for .223, .308, .243 (that was a surprise), 7.62x39, .22lr, or any of the popular Semiauto handgun rounds, there was plenty of ammo on the shelves. Using that logic, you can identify what will be in stock... The other common rounds that are not used in popular AR formats.
Since you're looking for enough retained energy at 300-400 yards, I would lean heavily towards the magnum chamberings with 30-06 as the floor and work my way up.
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Active Duty USAF 1994-Current