Compared to a 260 yes a 223 ai or 6x45 will run out of gas at 1000.

6x45 is a 223 rem necked up to 6mm. I think its a dumb idea overall.

223 A/I is a rem case with no body taper and a 40 degree shoulder. The result is a small gain in velocity, roughly 100 fps with heavy bullets. Depending on your elevation and barrel length, the A/I can stay supersonic to 1K with an 80 grain bullet.

Either of these cartridges can be easily converted from a 223 rifle.

Not sure where you live where Varget is so widely available. Wherever that is, I want to move there!

Bill

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