Originally Posted by Wikipedia

A Weaver rail mount (Picatinny type) is a system to connect peripheral accessories to rifles, shotguns, pistols, archery bows, etc.

The slot spacing in the Weaver rail is 3.8mm, while the slot spacing on the Picatinny rail is 5mm. Therefore, although they look similar, they are in fact quite different and not entirely interchangeable. While it is possible to attach a Weaver mount device to a Picatinny rail, the "slop" in the slot is large enough that a robust attachment is impossible for devices which click into the slots. Devices which are held to the rail by side screws or grips are usually interchangable.

Originally Posted by Wikipedia

The Picatinny rail is a bracket used on some firearms in order to provide a standardized mounting platform for scopes and other accessories such as tactical lights and laser sighting modules. The standard was first published by the Picatinny Arsenal and carries the official title MIL-STD-1913.

The rail is typically placed directly on the weapon's receiver, in the position normally occupied by the rear sights. Shaped in cross-section roughly like a wide T, scopes are mounted on the rail by sliding them on from one end or the other. In order to provide a stable platform, the rail should not flex as the barrel heats and cools. For this reason most Picatinny rails are cut crosswise, to give them considerable room to expand and contract lengthwise.

Originally used only for scopes, the rails were typically found only on larger caliber rifles. With the increased use of night vision scopes, they started to appear on smaller assault rifles as well, to the point today where they are replacing the original "iron sights" with versions that can be mounted on the rails.

Once the rails became fairly common, they started to be used for other accessories, flashlights for instance. This led to a sort of chain reaction, where practically every accessory is now designed to be mounted on a rail, including bipods, bayonets and laser sights. In turn, this had led to the introduction of "small rails" that can be fastened to the guns in various locations, and in some cases, entire grips with rails built-in on all sides. Short rails now appear even on shotguns and pistols.

Originally Posted by yanhchan

dude did you even search to forum?