I just sold my HK SL8. It has a 20" barrel (510mm) and a 1in7" twist (178mm).
In the manual HK even recommends using 52gr Sierra MatchKings.
Therefore I'm a) sure that a 20" AR15 with a 1in7" twist is not a bad choice and b) that a rather long barrel with a rather short twist will be a good for a wide range of bullet weights, at least from 52 to over 75gr.
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