What is the premier platform for big bore long range shooting? Best build kits?

Premier long range cartridge? So far I'm hearing 6.5 creedmore for ammo availability and price and ability. Is this correct?

What is the longest range you can get predictable and reliable hits with a simi? I'm sure at some point a bolt gun becomes king. At what point? 800? 1000? 1200?

That's a whole mouth full of mush, sir.

If you want to go on the cheap side of frugal, one way is:
A)  Buy an Aero Precision{AP} M5E1 builders kit when they are @ $300
B)  Buy an AP LPK minus TCG & Grip @ $28
C)  Get an AP LR @ $100
D)  Get an AP BCG @ $150
Schuyler Arms Company has great sales through the year.  The last half of 2018 was majiks!!!
E)  Buy a Proof Research 24" Carbon Fiber or SS 6.5CM.  Don't forget the Gas Tube to match, +2 and +3 for me.
Bartlein is Numbah Juan.  Scroll to the Chart and see who/whom is Numbah Too.
F)  Pick a Stock and call Clint at Heavy Buffers.
G)  Get an Adj. Gas Block of quality
H)  LaRue Trigger @ $87 and Grip of Your Choice

I've built 4 AP rifles from Schuyler Arms Co. Sales and can post Web Sales Receipts. Two rifles are 22" & 24" 6.5CM Ballistic Advantage barrels.

The other 2 rifles: 24" Proof Research CF 6.5CM and 22" Dracos Straight Jacket 6.5CM.

Paul Craddock has a 30" Bartlein blank being worked into a spiral fluted, .875" GBJ, threaded piece of 22" pipe for an ARMALITE B Series build spaced on my new ARMALITE bolt.

Now, if you want to just go for what gasser Mary Poppins uses for Ma Bell Calls...  [that's Practically Perfect In Every Way for those in Rio Linda]

Google the top 3 most respected 338 LAPUA gassers.  I'm partial to the KIVAARI® since I've known the guys for @ 2 decades, good people.

I agree with the quoted section very much.

If the goal is legitimate 'precision rifle' shooting, a semi can do it, but it's vastly more difficult to do than with a bolt gun. I dropped about $3k into a homebuild .260 AR10 off an M5E1 stripped upper/lower, and sold it recently after it's second barrel. First, most 6.5 bore AR10s are way over-gassed, nor is there any level of consistency in the gas port size or location, however if you want to 'shoot long' you've pretty well got to be shooting a 6.5/6mm etc. The .308 just can't do it well with the shorter barrels and projectiles you're limited to by the platform. So, you'll be dealing with levels of over-gassed that an AGB won't alone solve, the thing is going to beat brass up, have a relatively heavy trigger, and you've got a BCG the size of a 90s kia coming back at you every shot.

As far as caliber, there's really only one good choice. The 6.5CM. the sharp shoulder angle and the long neck make it so you can actually shoot 140 class bullets out of a magazine, and it'll be easy to find cheap/expendable brass vs. other options.

Regarding how far you can shoot it, you'd probably be going transsonic right around 1k (this is with most 140s at sea level).

I really liked the idea, and the look of my AR10. I didn't like to actually shoot it. As with any semi, you end up firing them faster, and banging through accuracy oriented 6.5s isn't exactly cheap. You're only getting ~2,000-3,000 rounds of accurate barrel life, it's almost never going to shoot as accurately (as effortlessly) as a bolt gun, and you'll probably be ~150-200FPS slower than most bolt gun counterparts.

If you're going to buy one, go GAP. If you're going to build one, at least make sure it's a DPMS Gen1 pattern (most are), buy an aftermarket bolt catch, and get the upper/lower/LPK/BCG from the same mfg.

My 2c.