I pulled targets once at Camp Perry with the inventor (Bill Alexander) of the 6.5 Grendel, and it was too new for me to ask good questions about it. Plus, it was not a 223 or 308, which were the cartridges I was shooting. I do have a very bad attitude towards cartridges du dour anyway, and I figured the Grendel would be another one. Well I was wrong about that.
One bud of mine had lots of issues getting his Grendel to feed from his AR, he tweaked the magazines a lot. Other buds have not had problems, but I think they were using single stack mags. This is a target round, so all it has to do, to compete in NRA competition, is feed five rounds from the magazine, or eight, for sitting and prone rapid fire. Most AR shooters shoot two, then reload with an eight round magazine, to complete a string of ten. From what I do know, it is an accurate cartridge, if you are firing it from an AR, don't be surprised if you crack your bolt lugs as the bolt thrust exceeds the 223. A bud of mine, his target AR came from the gunsmith with an extra fitted bolt, just for that future event. We talked about the minimum number of lugs necessary before a bolt swap out! Barrel life can be good if you down load the thing. Firing full power, full pressure 6.5 bullets will eat up the barrel. I am used to the 5000 rounds of accuracy life that a 308 barrel gives, so I am always surprised when I run into shooters who change their barrels around 1200 to 2000 rounds.
One shooter I met, replaced his 243 Win barrel around 800 rounds. He always fired full powered rounds and that ate up the throat of his rifle. He was trying to keep them all in the X ring at 600 yards, so his accuracy criteria is very demanding.
I don't care for any of these rounds. I am very happy with the 223 as a target round in the AR platform, it shoots very well out to 600 yards and barrel life is very good. As for a varmint/light deer round, I consider the 243 Win inferior to the 257 Roberts. I can push a similar, or slightly heavier bullet, to the same velocity, at lower pressures, which means my barrel will last longer.