Not sure what others have said, Didnt really want to spend the time reading the entire thread.
I have had good luck with a jp barrel in .223 wylde with a headspaced bolt. Got it on another forum for a very good price and its been great. While it not being an "ar10" caliber i can still tell you their quality is top notch. I've never really read anything bad on here from anyone for that matter either. Only input i have on anything besides factory barrels.
In terms of caliber, I started my long range game with a tikka ctr .308 had a 20" barrel. I was able to make hits pretty easy out to distance, but once wind picked up i noticed passed 800 yds it would be difficult for me since MY SKILL LEVEL WAS low. Fast forward to today, I have an Armalite .308 with an armalite factory barrel. Shoots around 1-1.25moa and im fine with that. Its a 16' gun that i run suppressed with an adjustable gas system. I dont really take it out beyond 600 yds, but it works very well out to that distance. I do have a 24" tikka in 6.5cm sitting in a chassis. It is very easy to shoot half moa with this gun(probably bolt vs semi, and tikkas barrels are more accurate). I will say going out beyond 800yds (shot out to 1200) is much easier with the 6.5cm. This has been beat to death on most forums but it is true that wind calls are easier. Again, i have no formal training in long distance shooting, just stuff ive picked up online. I use a kestrel 5700, and get my velocities from a labradar. The 6.5 just has better ballistics period.
So to make a long story short, if you want to shoot anything beyond 800yds i would probably look at the 6.5cm. If you want to shoot under that i would look at .308 for cost savings. I can find 168gr/175gr fgmm for around $16 a box. Whereas match grade 6.5 usually runs me about $24/box. Be realistic with yourself. Are you going to shoot so much that the cost savings apply? Are you really going to be skilled enough to use the extra accuracy out of a top of the line barrel? Lots of factors to consider. Also, if you are new to long range shooting most of the higher end barrel manufactures are going to shoot better than you can PERIOD. My guns shoot great, when I shoot great. My guns shoot like poop, when i shoot like poop. Im sure if someone with more experience and formal training were to get behind my guns they would shoot even better more consistently. The more you get behind the gun and shoot the better it will shoot with you behind it.
Good luck, and shoot often.
