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 either by sliding them on
from one end or the other, or by means of a "rail-grabber"
which is clamped to the rail by means of bolts, thumbscrews
or levers. 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
displacing the original "iron sights" of many weapons with
versions that can be mounted on the rails.
Once the rails became fairly common, they started to be used
for other accessories, such as flashlights. This has led to
many accessories now designed rail-mounted, including
bipods, bayonets and laser sights. In turn, this had led to
the introduction of "small rails" that can be fastened to
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.
