I can't see the sense of a 20" heavy barrel except maybe for a swat cop to carry in his patrol car. The longer barrel will give you a little free velocity, noticeably less muzzle blast, and a longer iron sight radius if you took up Palma shooting or some such.
By the way, 5R stands for 5 groove, Russian pattern, and is based on adaptation of the rifling plan of AK74s captured in Afghanistan to US calibers and manufacturing practice. The Russians got it from a British design of about a hundred years before.
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