At this point, ignore Brand, because whoever just might not be building the gun you need to match up. Put the "AR15" part aside, it's obviously a precision varmint long distance gun.
1) The caliber is going to be intermediate to medium
2) It needs a longer barrel to get all the energy and fps to reach out at it's maximum ranges, which can also reduce bullet drop and make holdover easier.
3) It's more important to have an adequate optics platform, and ignore a lot of the short barrel tacticool stuff. CQB is the opposite of what is wanted, different thing altogether.
First, the barrel, a good name brand quality stainless target barrel over $300. If it says HBAR it means nothing, that's just a standard 2MOA with material not machined off. No chrome, stainless. 20" is all you need, rifle gas.
With that in mind, it needs the A3 upper, but a full length railed handguard will be basically useless because you can't bridge the scope mount to it - a cantilever mount is pretty much mandatory. A free floated barrel will help, because the bipod mounted to it won't be moving the barrel around. That only takes one short piece of rail, so the tube is actually that - no integral rails at all.
Stock, either A1 or A2, try some out for length of pull. Don't worry about an ubercool tactical stock until you've shot it regularly in matches and the only guys better than you have them. Then you will have enough experience to understand why it makes a difference, and whether a $250 stock is justified for the less than 1/4MOA it offers. A shooter actually has to be 1/2MOA good to even see it.
After that, mount a scope on it in the 4x12 range that costs at least 1/2 what the rifle did, minimum. Most spend more - more than the rifle.
With that rifle, start reloading. The money you saved on the stock and fancy quad rail will literally buy a turret press reloading kit. The reloading kit will let you load ammo that shoots 1/2MOA, where even the best premium target loads across the counter might only approach it. Reloading is way cheaper.
Unfortunately, the average beginner simply doesn't have the skill to shoot long range precision much at all. It takes developing their understanding, reading wind, knowing the bullet drop of their best load, and practice, practice, practice. Paid sharpshooters actually do that more than anything, in all kinds of weather and under some pretty horrible conditions, for fun. If it looks nasty out, they go.
Like everything in llfe, it can't be purchased, it's bought by paying your dues.
