Handguards and Proprietary Attachment Systems

Finding your way through the options of handguards.

Hand guards are a part of AR15s that extend forward from the upper receiver. Once the barrel and gas tube are installed  the handguard will slip over them to provide not only protection to the gas tube and barrel,  but also provide a space for your hand to grip the front portion of the rifle, while keeping your hand away from the heat produced by the barrel while in use. Also providing space to allow connection of various attachments.  Not only are hand guards a vital part for accessory attachment but they play a large role in the overall appearance and functionality of the rifle. While aesthetics are relatively subjective, if you were to ask most AR15 owners, they'd probably say they liked the way their rifle looks as well as how it functions.
There are 2 main types of hand guards: drop-in and free float. Drop in hand guards are used with ar15s that utilize a front site post gas block. It provides a space for the front most portion of the hand guard to lock into and attach. Drop in hand guard also connect near the upper receiver, by exposing a small area covered by the delta ring. Which is pulled back to allow the hand guard to attach.   Generally with drop in hand guards, they will be a predetermined length based on the gas system length however this is not always the case. Ingenuity once again has brought us a number of options of hand guards that extend past the site post gas block, for more hand space and room for attachments.
Free float hand guards are utilized with Ar15s that are equipped with low profile gas blocks.  Connected only by their attachment to the upper receiver. These have significantly gained in popularity in more recent years.  Handguards come in many different lengths and diameters, usually depending on the gas system of the particular rifle, user preferences and sometimes even barrel diameter.  Also, Some hand guards may need to be a certain diameter or length to support the attachment of a suppressor/silencer.

Proprietary attachment systems: There can be only one...Or two.
There are 2 major proprietary systems for attachments on handguards today. keymod and mlock. Keymod has been around longer, since 2012. Where whereas in recent years, despite its later release in 2014, mlock has without a doubt taken over the market in popularity. Key mod handguards and attachments are still available. Keymod and mlock can be distinguish by the types of cut-outs in the hand guard, which facilitate the attachment of accessories. Keymod cutouts are shaped like keyholes. Mlok cutouts are just flat or straight the whole way. These systems add yet another layer of customization ability. Picatinny rails can be used only when and where needed. Allowing the user to customizing weight and hand space. Increasing the ergonomics of the system. 

It is critical to take note of which proprietary system your hand guard employs, so that when purchasing attachments you can be sure to have a compatible system.