I agree with the fellas above. 4” is usually very noticeable, while 2” is often still noticeable. 20” is my cutoff for a short 7mm-08/308, while I prefer another inch or two for smaller bores with that case capacity. At the bench not long ago, I mixed in a magnum weight 22” .308 sporter with some various carbines and magum-chambered sporters. It was a downright sweetheart to shoot, both to the ears and the shoulder.
