Because you ask i think you should start with a .223 wylde 20" precision stainless barrel. It will take you out to past 500 yards with the right bullet and glass. It is a relatively cheap caliber to learn on and widely available.
Once you've understood the ballistics of the .223 remington/5.56 nato and have outgrown the caliber and mastered the fundamentals of precision shooting, then I'd do some research into other calibers and then go for more
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