Been there, Done that.
Get a box of each so you're not stuck with a pile of ammo that your gun does not like. Try to establish a shooting position that takes as many variables out of the equation as possible. If you're shooting off of a wooden block with a towel for a rear bag.....you may need to reevaluate what you're doing. You want a solid, repeatable position. 5 rounds per group....4 groups per box.
With that barrel, if you're good, I would expect .5moa or better. Any ammo not holding that, move on.
Hornady Match 140gr ELD. I keep trying other ammunition and have yet to find anything more accurate. They also make other bullet weights, try a box of each.
Hornady Black 140gr Match. This ammo is very accurate as well and can be routinely found for under $20 per box at Acadamy Sporting Goods or online. Hornady American Gunner 140gr seems to be the non Acadamy equivalent.
Prime 130gr Match. A solid choice if your gun likes it. Mine did not.
Copper Creek Cartridge Company. You can go hog wild trying all the different bullet brands and weights. Their .223 77gr Berger ammo shoots the best out of my JP 20" precision gas gun. They are probably the most expensive option so heads up, but they make great, accurate custom ammo.
Defender Ammunition Company. They have 2 different 6.5 CM options. Their 69gr SMK .223 ammo shoots great out of my JP....probably just as good as the Copper Creek at around half the price per round.
Desert Tech Premium Match 140gr. Gets good reviews but not widely reported on. May try some.
Good Luck.
Hornady Match 140gr ELD out of a 27" Hawk Hill Barrel. 5 rounds @ 100 yards. I get pissed when every group isn't like this
Is that normal?