With 55 grain bullets you will lose 170fps and its noticeably louder when comparing 16" to a 20" barrel. I was thinking more in terms of efficiency and practical use. I don't think many buyers see an AR as anything but another cool toy. As for cheap, it wasn't long ago a new SKS was under $100 and ammo was less than 10 cents a shot.
I would venture the say the vast majority of ARs built are less than 20” for many reasons and would argue that if it didn’t constitute an SBR, people would go even shorter, regularly. 170fps is negligible to many people considering that’s only what, 5-8% drop of total bullet velocity using round numbers? In general, It hardly affects one’s ability to shoot nearly as far as they really want. Maneuverability, weight, use case, length with suppressor, etc etc. I would argue that a 16” is much more practical for an all around use rifle than 20.
As smitty just said, that’s the beauty in AR’s. Build whatever you want, they all work great. If you want to talk about something that impractical, you should see my 8.5” 300BLK upper that shoots only subs for the range

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