I had Kirby Allen build something similar to what you are talking about for me last year. Rather than list it all here you can read about it "here"
if you’re interested. If I had it to do all over again, and wanted an extreme gun built to handle any long range conditions, I would duplicate the same gun exactly but that is just what it takes to make “me” happy. You need to decide what level of components you want to incorporate and what level of performance you expect and what level of expense you are willing to accept. I’ve had, and worn out, a lot of 7mm, 30 cals and 338s and would still duplicate what I have. I’ve found no fault with it and am comfortable putting the crosshairs on something and tripping the trigger. Kinda makes me feel warm and fuzzy inside when things happen big time, quickly, wayyyyy down range.
Some generalities, from my point of view, would be:
• Quality action, like an accurized 700 or full custom action, sized to handle the round of your choice.
• Trigger can be reworked factory or aftermarket, set at a weight that is safe and comfortable for your shooting.
• Cartridge and caliber dependent on your personal feelings but one that is able to handle heavy for caliber, high BC bullets at a velocity that gives you great down range performance.
• Use a quality, heavy for caliber high BC bullet in a design that will handle the target or game that you shoot. Don’t spend thousands on a gun and try to save a few bucks on bullets.
• Laminated or synthetic stock of a style and design that makes for a comfortable gun that you can shoot under all conditions.
• Top quality barrel in your chosen caliber with a twist rate for the above bullets, probably in a minimum of 28” but, for me, preferably 30”.
• If recoil in your chosen setup is an issue, then a quality muzzle brake. I like Holland QDs.
• Good quality scope with externally adjustable turrets containing quality glass and a total package that gives you repeatable dialing day in and day out. Choose a range of magnification and a reticle that will handle the targets you intend to shoot, at the ranges you intend to shoot them.
• Last, but most important of all, choose a smith that knows what he’s doing and has a proven track record with easily checked references. It doesn’t matter how much money you spend on components, if they aren’t put together right then you spent all of you time to build an expensive 100 yard club.
Good luck with your build, spend a lot of time in the planning because that will determine the end product that you take to the field each time. Remember that there are factory guns that will handle your 800 yard shots just fine if properly chosen and set up.
If you decide you want a really precise long range hammer then don’t skimp on important components because I guarantee you will be unhappy for as long as you own the gun if, in the end, the quality and results of each shot get compromised because you cut corners.
There are literally hundreds of combinations that will accomplish what you want. You need to decide on each component, based on your likes, needs and pocket book. Play the what if game and build your dream gun on paper. Play with some ballistic programs and see what kind of performance you will get with different combinations. Price it out, change the components around and price it out each time. When you are happy with the components, quality, price and performance, then buy it or have it built and spend a lot of time sending rounds down range.
Dead is dead and there are countless calibers and rounds that will accomplish this for you. Good luck with your planning and I hope that whatever you choose works well for you and that you are happy each time you trip the trigger.