18" is fine, but you need a stainless match barrel. I second John Hollinger as a great source. I've built two 18" guns with his barrels. But, I shoot 8" at 400 and 12" at 600 because I like to hit it most of the time. 4" at 600 with 5.56 is just sick. If you can spot your misses, it might be a fun challenge. If you can't spot them, it's just frustrating.
Remember, putting 5 shots into a 4" circle at 600yd is very hard. Putting them all into THAT SPECIFIC 4" circle is damn near impossible. The F Class 600yd 10 ring is 6". And that's bolt guns firing 6.5x284 and 6 Dasher and stuff. The 9 ring is 12", so if you can hit a 12" plate reliably, you could score real well in F-class.
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