Modelling it in QL, 120gr TTSX, 40.0gr H4895 (AR2206H), 3.150" OAL.
22": 2834fps
21": 2802fps (-32fps)
20": 2767fps (-35fps)
19": 2691fps (-76fps)
18": 2612fps (-79fps)
17": 2528fps (-84fps)
16": 2438fps (-90fps)
Based on that, and that alone (i.e. no real-world testing), I wouldn't go less than 20".
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