The lines started blurring a long time ago when a certain American military unit started (re) realizing that they weren't the top shooters, nor close. The only difference between paper, steel, and flesh is the emotion we put into it- the guns don't know the differance.
There's about 30,000 words here for those with an inkling......
From the first Carbon fiber freefloat handguards to the MK12, to the KAC MRE, to the MK12 Hollands, to the present Block II M4's, there's been a steady progression..... Irons, Aimpoints, EoTech's, magnifiers, 1-4x's, 1-6x's, etc....
Even the Marine Corps is coming on board- looking at 16 or 18in barreled M16's with collapsible stocks, extended freefloats rails, Giessele triggers, and muzzle brakes. Yes- muzzle brakes.
As for barrel length make mine 14.5in please. I can do 16's, but all considered the 14.5in is the sweet spot...
An extended freefloat rail is the biggest upgrade that can be done to an AR/M4.
And is the first thing I would do to one. Yes- before a trigger, or even an optic.