Scope mounts are rigid implements used to attach (typically) a telescopic sight or other types of optical sights onto a firearm. The mount can be made integral to the scope body (such as the Zeiss rail) or, more commonly, an external fitting that clamp onto the scope tube via screw-tightened rings (similar to pipe shoes). The scope and mount are then fastened onto compatible interfaces on the weapon. Words such as mounts and bases are used somewhat loosely, and can refer to several different parts which are either used together or in place of each other as ways to mount optical sights to firearms. Attachment interfaces for scope mounts vary according to weapon design and user choice. Traditionally scope mounts are fastened onto firearms via tapped screw holes (usually on the receiver) and/or clamps (onto the barrel or stock). Since the mid-20th century, dovetail rails, where the mount is slided over a straight dovetail bracket with an inverted isosceles trapezoid cross-section and fixed tight in position with clamping screws, became more common due to the ease of installation and removal. Later, the hexagonally cross-sectioned rail interface systems such as Weaver rail became popular and was later modified into the Picatinny rail in the early 1990s, which became the standardized military-use mounting interface for NATO troops in 1995. The Picatinny rail was officially replaced by the metrified NATO Accessory Rail for military use in 2009, although it remained popular in the civilian market for both scope and accessory mounting. Scope mounts can be either one-piece (a single implement with multiple clamping rings) or multi-piece (usually two or more individual scope rings). These mounts are usually fastened with screws to specified tensions (which warrants the use of torque screwdrivers), but sometimes they are manually tightened via thumbscrews, and may even have Quick Release (QR) designs. As of 2020, the Picatinny rail is arguably the most widespread scope mounting standard for new firearms, although there are many proprietary and brand-specific types of mounts that can either be used with Picatinny rails, or as completely different design alternatives (see the section on Link between scope and firearm). Scope mounts may be offered by firearm and scope manufacturers, or bought as aftermarket accessories. Optical boresighting, also referred to as "eyeball boresighting", as the name suggests, involves using the naked eye and the bore axis of a firearm to dial in the sights of a firearm. Based on the type of firearm the procedure for optical bore sighting will vary. For AR style rifles the upper receiver can be dropped from the lower receiver and the bolt carrier group (BCG) can be removed exposing a clear line of sight through the barrel. With the upper receiver fixed in place you can then co-witness through the barrel and the sights on the top of the rifle, while doing this you can visually align the bore axis with the sights on a desired target. For bolt actions rifles the process is the same aside from the ability to remove different receivers, but the bolt can still be removed to view through the barrel. Once you are able to align the bore axis and the sights on the firearm you should then be able to at least get shots on or close to the target. With the rifle reassembled you should be able to put shots down range and make more fine-tuned adjustments. A drawback of using the naked eye to boresight is that it can be difficult due to the tunnel vision and lack of brightness, especially with longer-barreled guns. A more advanced method of boresighting uses a collimator boresighter, an optical attachment similar to a scope sight, which fits onto the muzzle end of the barrel via a bore-diameter arbor and projects a grid pattern onto the sight picture.[2] Using this method, the normal sight (which is fixed to the receiver) and the collimator (which is fixed to the barrel) can be sighted to match. Most collimators have grid patterns for rechecking the zero after the barrel is sighted.
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