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Optics

Rail Crazy: Picatinny Rail Basics

by J. Guthrie   |  September 23rd, 2010 4


The MIL-STD 1913 Picatinny rail is a dirt-simple system that has revolutionized the way we attach optics and accessories to firearms.

The whole world has gone rail crazy, and with good reason. The MIL-STD 1913 rail developed at the Picatinny Arsenal in New Jersey is not overly complicated, outrageously expensive, or even hard to manufacture. It is a simple accessory-mounting platform that has revolutionized the way we attach all manner of things–optics in particular–to our firearms.

My initial research into the Picatinny rail revealed a world cloaked in myth and mystery; there are a lot of claims and counterclaims about who came up with what when. It was as if I was searching for the optic editor’s Holy Grail. Then I found the Picatinny Arsenal website, called the number to a very nice public affairs office, and had the chance to talk with Gary Houtsma, a mechanical designer who spent the last 32 years of his career at Picatinny and actually was the lead project designer on the MIL-STD 1913 rail way back when.

Shortly after the U.S. military developed the technology to fight at night, the quest for a standardized mounting system began. All sorts of night-vision and night-aiming devices were being developed for individual and crew-served weapons, and those systems needed to come off when the sun came out yet put rounds on target when they were reinstalled at night.

Every company had its own way of attaching the aiming devices, and each required its own mounting platform. Frustrated with all the configurations and dimensional variations of the different dovetail-mounting systems, the U.S. Army tasked the arsenal with developing a standardized mounting system.

“We started on the project in 1992,” Houtsma said. “Most companies were using a rail-grabber of some sort, but they were tight on some rails and loose on others. No one ever had standardized dimensions. We brought in different weapons that had rails from the weapons bunkers at Picatinny and even went out and picked up rails from sporting-goods stores. We sat down, measured the distances between a randomly chosen datum on the four 45-degree-angled surfaces and a height from the datum to the top of the rail on the 20 or so different rails, and came up with an average set of numbers.”

Houtsma took the dimensions by committee over to the facility’s production side and requested they design a dimensioning style so the rail could be easily produced and inspected. At a first glance, they commented that the rail was exactly like the one on a 105mm howitzer, just scaled way down. Under recoil, a howitzer’s tube slides up and down a dovetail rail for repeatability. Turns out, it is important to put really big bullets in the same place miles away every time, too. The arsenal just coopted the howitzer rail production and inspection procedures for the new optics rail.

“We sent the finished product over to Rock Island Arsenal, where it was reviewed, and the rail was then sent out to all the different services for comment,” Houtsma said. “The final design was then sent to our experts in the technical data section to determine if it should be a standard or a specification. They determined it was standard, not a specification, and did the final drawings. The rail was adopted and fielded in 1995.”

The rail concept was not developed by Picatinny, merely refined and standardized. The Weaver rail long predated the 1913 rail but has several key dimension differences, most notably the width of the recoil lug/cross slot. The Weaver slot measures 0.180 inch in width, and the 1913 has a 0.206-inch slot. There are no Weaver standards for slot spacing. Standardized spacing was required for the Picatinny rail since some military optics at that time had two recoil lugs. The 1913 slot-spacing center is standardized at 0.394 inch. Most Weaver rings will fit a Picatinny rail, whereas some but not all Picatinny rings will fit a Weaver rail. It mostly depends on the dimensions of the recoil lug.



Picatinny rails are usually numbered so that optics can be attached at the same point after removal. Failure to use the same slot can result in point of impact shifts.

The standardization was a windfall for the firearms industry. Manufacturers of guns, rails, bases, and rings all were on the same page, saving a lot of work and machining time. Shooters could now buy one mounting system that worked on a myriad of guns. The 1913 rail brings peace and happiness to the lives of optics editors everywhere since they swap scopes and mounts as often as most folks change their underwear.

Houtsma said there were no standards issued for the rail as for maintaining zero; those standards are met by the mount and optic. High-quality mount manufacturers take return-to-zero very seriously and have developed some ingenious methods of making sure the scope and muzzle are pointed at the exact same spot every time.

For example, LaRue Tactical makes some really nice optics mounts and guarantees absolute return to zero. On the firm’s lock-lever mounts, a cam attached to a locking lever allows the mount to exert pressure on the rail from three directions, pulling the mount down onto the rail. The locking lever is adjustable, which allows the shooter to correct for rail tolerance, stacking, or a manufacturer’s sloppy machine work.

“It is important that the mount is attached to the rail properly,” said Cody Mitchell, a customer-service representative at LaRue, describing the lever-lock system. “You should start feeling contact when the lever is at a 45-degree angle to the rail. The lever should be tight to close and tough to open. That delivers the proper amount of force to hold the mount in position.”

When the mount is sitting in the recoil lug/cross slots before the lever or locking nuts are tightened, there will often be a little play. This front-to-back wiggle is a result of the mount’s recoil-lug dimensions. They are slightly undersized so that they go into and come out of the slots easily. The mount should be pushed forward so that the lug contacts the slot’s front wall, especially if the mount is subject to heavy recoil or rough handling.

All Picatinny rails are supposed to be numbered, allowing the user to remount the optic in exactly the same position every time. Failure to do so can result
in a subtle shift in zero, which could be a huge issue if the optic is paired with a precision rifle. I have a favorite AR carbine that has a complete suite of optics for various applications. Each optic has a dedicated mount and a note card that helps me remember where it sits on the rifle’s 1913 rail.

The rail has caught on, obviously, and is now used to hang bipods, lasers, and flashlights–the list is endless–on rifles, shotguns, pistols, and belt-fed machine guns. Regardless of the accessory, it sure is nice to know there is a system for optics that really works. Thanks to our friends at Picatinny Arsenal.

  • Dolph

    Can anyone tell me what the large threaded holes near both ends of the rail are for?

    • PTR guy

      If its on a weaver rail it is for stanag scopes like the zf optics for G3's

  • bladder456

    Yeah, to attach them to rifle!

    • rocky

      on my quad rail the large threaded holes are on top. they would smash the gas tube if you ran a screw in to them . can’t be for mounting to rifle

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