Okay, now let me explain from the most widely known guns.
AK 47 (Avtomat-Kalashnikova 1947)
Here is the modern look of the AK 47. It fitted with RIS, Rail Mount, attached with Mepro M21 Mars Sight, the full body modification from Krebs.
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Colt M4A1 Carbine Assault Rifle (AR-15)
The M4A1 carbine rifle, or known as the AR-15, is the best version from all carbine rifles, which this gun is very precise, also recoiless. it cartridges the 5.56x45mm NATO, is a cheaper bullet than the 7,62x39 ammo. And this rifle was very modifiable. This assault rifle has the similar system to the classic M16, but the piston is getting shortened but still keeping its great range (500 meters) the short piston system give more rate of fire, so this rifle was one of the universal rifle in the world.
Here was the look from one of the M4 modifications. Magpul arsenal has made a great custom modification for this rifle. It uses RIS fore grip, Magpul PMAG, TAO1NSN ACOG Sight, Magpul MOE stock, and also custom barrel from Daniel Defense.
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Bushmaster ACR
Bushmaster ACR was known as one of the world's best assault rifle, because its accuracy and power belongs like not to be an assault rifle, it is a powerful weapon! It uses the 55.6 NATO (5.56x45mm) cartridge, but its power was very impressive, causes by technology of the mixing the both tilting bolt and Kalash's traditional gas operation system. But, this gun is not a cheap weapon, only some special forces that use this weapon.
ACR is a really modern weapon, the standard version is already placed with RIS. This modification includes Trijicon TAO1NSN ACOG Sight, Magpul Rail Vertical Grip (RVG), Surefire X400 laser sight and flashlight, Foldable-extendable version ACR buttstock. the foldable sight uses Magpul MBUS Pro. The pic comes from armslist.com :D