Everything written above is true fact the question is how deep down the rabbit hole do you want to get? want to kill the mile then just get a 300 win mag and call it a day you will still have a good amount of work right barrel bullet combo powder etc .....where I live there are long distance shoots all the time chances to hit the mile quite often and longer it is a thing called the boomer shoot where you shoot Dynamite out to 13 or 1500 yards in 6 inch targets..long distance is a thing here I am new to it still learning you will see everything from 6mm to 375 and every kind of gun in the open class they also have the hunter class the gun must weigh under 14 lbs.
my recommendation is just like 98 said think about how much you will use it , are the places to shoot readily available?are you willing to spend 5 grand on a gun just to ring steel out to a mile? if yes then the world is your oyster,300 win mag,7mmPRC 300 PRC 284 win /with 180 grain bergers, 6.5 x284 7mm saum and a tomn of other calibers will do it if you just want to build a fun long range gun then I have to agree keep it simple go 6.5 CM or 260 rem. but this rabbit hole is deep! I mean Deep!