BudsGunShop.com - What type of rail

The handguard is the BCM MCMR-15, the top is Picattiny rail and the sides and bottom are all M-Lok slots. bravocompanyusa.com/BCM-MCMR-15-M-LOK-Compatible-Modular-Rail-p/bcm-mcmr-15-556-blk.htm

The rails on both the upper receiver and the hand guard are standard picatinny rails, The Picatinny rail (/ˈpɪkətɪni/ or /ˌpɪkəˈtɪni/), or Pic rail for short, also known as a MIL-STD-1913 rail or STANAG 2324 rail, is a military standard rail interface system that provides a mounting platform for firearm accessories. It was originally used for mounting scopes atop the receivers of larger caliber rifles. Once established, its use expanded to also attaching other accessories, such as: iron sights, tactical lights, laser aiming modules, night vision devices, reflex sights, foregrips, bipods, slings and bayonets.