mounted to a rifle in perfect parallel to the barrel, and to ensure the cross hairs indicate a point of aim (POA) exactly at a bullet's point of impact (POI) at a known distance, a small variation of even one quarter of one degree can cause massive problems at longer ranges. The locking bar holds the mount in a perfect 90 degrees to the rail system, whereas a non-locking bar system can cant to the left or right. This canting (sometimes called jamming of surfaces) is caused by not matching the clamping surface perfectly to the rail. When tightened down, stress exerted on the base can cause the scope's POA to be off from the POI by as much as several feet at 100–200 yards, and gets progressively worse with greater distance. Lower grade materials used in manufacturing of scope bases, inconsistent design tolerances from one manufacturer to another, and other factors can cause twisting stress and cause the mount to move out of parallel with the rifle barrel. The locking bar system allows for even stress to be distributed and prevent canting of the scope mount. Another form of scope canting is caused by the rings themselves. Many Weaver-type mounts (including many Picatinny-type scope rings and even the Redfield Type) have either two or four screws on top of the scope ring that hold the scope in place. Both the Weaver and Picatinny clamp systems have a screw side and a clamp side. With the two-screw style, the ring usually aligns well but does not have the strength of the four screw system. Many times, when tightening the screws of the four screw type, the scope can twist in place, causing misalignment. Tightening the mounts in an “X” pattern and usually starting opposite of the outside clamp side; holding the scope slightly to the opposite side while tightening the screws usually allows for proper alignment of the scope to barrel parallel. Compatibility with Picatinny The military standard, MIL-STD-1913 "Picatinny rails" date from the mid-1990s and have very strict military standard dimensions and tolerances. The Picatinny rail has a similar profile to the Weaver, but the recoil groove width of the Picatinny rail is 0.206 in (5.23 mm) versus 0.180 in (4.57 mm) of the Weaver rail/mount, and by contrast with the Weaver, the spacing of the Picatinny recoil groove centers is consistent, at 0.394 in (10.01 mm). Many rail-grabber-mounted