You run the minor risk of the headspace going off. A very slight chance that increases as you use it more. It's hard to tell exactly when that might be but as I mentioned before I did have a low count barrel that closed on a no-go after a few rounds. It may still be safe to fire for a while, I don't know.

Other than that, you can swap willy nilly as you want but the only issue is that you're wearing the bolt slightly differently. Just by how much is not a known factor and may never be an issue in the long run but it makes no sense to not keep running the "better" of the two as you'll probably run a even greater chance of it wearing out sooner by swapping it out continuously because as you shoot, the bolts are mating to the upper, so to speak and two bolts wearing might, ever so slightly, wear differently.

The thing about ARs though is that they're quite easy to mix-match but I think in doing so you're adding a cumulative breakage that might not be scientifically provable and may never be proven with any repeatable result since there are so many tolerances and just how much and what ratio might play into it. Regardless what is known is that mating a bolt and barrel is better and safer since you're dealing with a known factor but when you swap bolts and wear in two bolts, there's no way to know what, if any, effects it has.

Build a basic carbine for whatever and second for precision.

Your ammunition selection is what's important at the 600 yard line. Try 80 grain SMK's seated @ 2.470" in a .223 Wylde chamber, or seat them @ 2.550" in a5.56mm NATO chamber. Load them one at a time, as required by the rules.

22.5 to 23.0 grains of H4895 or IMR-8208-XBR

23.5 to 24.0 grains of Varget, RE-15 or VihtaVuori N140.