Really don't think you'll see any appreciable velocity gains by staying with the longer barrel. The 300 was designed for short barrels. Go as short as legal I reckon. Make it light fast and handy.
My ar15 has a 12.5 inch Barrel and is about 15mm over minimum legal length. I use it for NI bush hunting and can't think of any situation where having an extra 50-100 fps would make any difference to what I'm shooting. If there's slips or big gullies i just tale a bigger gun
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