$6,000 and a favor, give me some recommendations on what to get.

I think you may come out ahead if you go the route of buying accessories for your existing guns. I'm no FFL, but I'm gonna venture a guess that the mark up is greater on accessories versus firearms. Bipeds, slings, brakes...if you get more than you need, you can always sell a couple items for a profit.
 
DTA HTI best way to go for higher caliber. I have a Barrett MRAD 338 and love it. However, as mentioned, with all accessories and good scope-March on mine-you'll have more than $10,000 into it. Ammo also more expensive than 50 cal. DTA gives more flexibility with multiple calibers and only one scope and suppressor needed, if you plan to get the latter. Really no cheap way to shoot a mile or more. However, a few higher caliber rounds on steel at those ranges is a blast and only a few rounds needed each session once you can hit fairly consistently. In summary, any high caliber endeavor will most likely run $10,000-15,000 at a minimum, so be prepared!!! I hope you're single or your wife is either very understanding or clueless. Good Shooting