Civilian clone Rifles are the largest adapters, while the Crossbows, Hunting Rifles, shot guns and Handguns have started to come from the factory with rail sections ether attached and/or made structurally as part of the actual Firearm. HMGs have started to include and use rail sections and options for attachments of optics. Police and Military style firearms may include Pistols, PDWs, Carbines, Rifles, Submachine guns, Light Machine Guns, and Heavy machine guns. Most modern military and civilian semi-automatic Firearm have rails that may replace original parts. With equal and even more use due to additional novelty items is the AR-15. The firearm often associated with, and that has benefited from rails is the M4 Carbine & M16 family of later models/variants -A2 -A3 & -A4. Original rails were a raised metal strip with the sides under cut, less standardized than the dovetail design, to allow hardware to slide on and be secured by means of compression only. Rails as a term has evolved to cover both the actual rail pieces and the styles of handguards (or forearms) that are made with rails as the external surfaces. Tactical usage and Shooting sports have both benefited from the extra options provided. Along with the ability to switch different items at different placements due to varying eye reliefs on gun sights, scopes & optics. An advantage with the multiple rail slots is the moveable positions to adjust for optimal placement of each item for each different user. An example of just a few options that may be used depending on a mission's need for the M4 is SOPMOD, amongst many other accessories like sling attachment points (which may be ether ring, loop and/or quick detach mounts using push button style hardware) to name just a few of the many options. Rail systems on firearms are straight mounting brackets (usually made of strips of metal or polymer) on the gun's receiver, handguard or fore-end stock to allow sliding/variable-position attachment of optical sights and accessories such as tactical lights, laser sights, vertical/angled foregrips and bipods.

The M4 carbine with a Picatinny rail system on the upper receiver and 4 sided handguard, showing a few possible accessory options such as a Grip Pod a type of Vertical forward grip, and an M68 CCO sight

Keymod rail section beveled