6.5 Grendel is pretty scarce. I had picked up some Wolf FMJ factory, which I"m using to generate brass for reloading.
Regarding getting another barrel (non-restricted), you might want to reconsider.
I don't think putting a better than stock barrel on is going to get you much more in terms of accuracy.
The stock 7.62X39, non-restricted barrel, is supposed to get pretty decent accuracy (2MOA or better) even using military surplus, and I'm sure the .223 is probably in the same ball park.
It goes back to your reason for wanting the aftermarket barrels in the first place.
I went the other way....shorter than stock...in order to make the XCR quite a bit more 'handy' doing CQB type of shooting.
I'm limited to using the shorter barrels at the range only, but that's not an issue for me.
I'll probably do the same thing in .223, and then I'll have my choice of shorty .223, 6.5Grendel, and 7.62X39.