For F T/R 600yd MR, I use factory rifles, either Savage 11VT .223's, or Stag Model 6 Super Varminters.
They are all NATO 5.56 chambered, the Stags use 1:8" twist, the Savage has a 1:9" twist. I load my cartridges in PPU brass which supposedly has thick necks, with Hornady 75gr HPBT-Match bullets, loaded to about .005" short of magazine length in a standard STANAG AR Magazine. I run about 23.5gr or 23.7gr of Varget and CCI BR-4's. These rifles approach .5MOA at 100yd, still tuning the load, but they all shoot the same load well enough to be competitive. 600yd is no stretch for the load.
Custom is really nice, but factory rifles can still get all the job done for all but the very best shooters.
If I could shoot like them, I'd be buying custom, but by the time it shipped, I'd have a 50/50 chance of dying of old age. I'm about to turn 72.
You'll notice that at no time did I mention the word freebore. For my needs, what I have from the factory is doing what I want it to do, so I don't look the gift horse in the teeth, I take what I got and run with it. So far it's quite satisfactory.
Learn what you have to learn, but don't be afraid to try what you already have.
I've been shooting factory Savages since the mid 1990's, and the Stag 6's since about 2004. I don't win, but I'm not a winning caliber shooter to begin with. I get to run with the big dogs, and we all get to be happy.
My Granddaughter shoots the Stag 6 in 600 yd F T/R, and I just built another from the factory kit for myself. I use the Savage 11VT .223 at 600 for F T/R, and may use my Stag 6 for it someday, too
Meanwhile, no gunsmithing waits, everything I have was built by me, the two F T/R rifles (11VT .223, 11VT .308) are up and running, and I have a proven 1000yd F Open Lothar-Walther 28" 1:8" barrel to spin onto the .308 for the 1000yd line. Total cost including replacement stocks and optics, including the L-W barrel, between $2500-$3000 to shoot all three F Class disciplines, and only the barreled actions are original factory. The rest (Choate Tactical stocks, Mueller 8-32x44 Optics, Vortex and EGW mounting) comes from Midway (mostly) and Amazon. Total personal time investment replacing stocks, optics, less than a day total.
Or..., I could also just leave the .260 barrel on the near-ancient SH Ghost Dancer 260 custom Savage, but that would be cheating... I'm just about ready to mount a vintage Weaver T-24 on the Dancer.
It's not about the money. A frugal shooter with a little courage to try new home gunsmithing things can do it all. Give a good shooter basic equipment that has the potential, and good things can still happen; just without all the extra bucks and waiting time. Simpler probably has fewer bugs, too.
Greg