Is Your Rail Truly Mil-Spec?

by
posted on March 28, 2012
truly.jpg

Many of today's manufacturers of AR-15/M16 parts and accessories claim to be selling "mil-spec" parts. Unfortunately, either they don't truly know what military specification is required for a particular component or feature, or they don't care to try and hit the spec because doing so would increase production costs beyond an acceptable limit.

A great example is the M1913 accessory rail, more commonly called a Picatinny rail after the Picatinny Arsenal where much of the development work was carried out. But is every rail called a Picatinny rail truly mil-spec?

If a Picatinny rail is mil-spec it must be manufactured to certain specifications and/or dimensions with tolerances.

An example of the difficulty measuring for mil-spec is the .835-inch dimension is required to be in position to datum –C– within .006 inch at maximum material condition. The maximum material condition requirement means the .835-inch dimension can be out of position as much as .006 inch—if it is .835 inch. If, in the manufacturing process, this dimension is made smaller than .835 inch, it must be closer to its required position.

So you see, this kind of dimensional tolerance can be difficult for the average guy to check.

However, some dimensions are easier to verify, like those on the profile of the rail—the .367-inch minimum, the .835-inch maximum and the .617-inch maximum.

Cross slots should be .206 inch (with a tolerance of +.008 inch/-0 inches) and be on position to datum J within .006 inch at maximum material condition. This means the slots can vary more in their .394-inch center to center position if they are wider than .206 inch with a maximum width of .214 inch. Depth is held to .118 inch with a tolerance of +.008/-0 inch.

If you want to check these dimensions, use calipers (preferably an electronic model) to measure the width and depth of the recoil grooves. The spacing is a little more difficult, however, once you have established the widths of the recoil grooves, you can measure the inside distance between two grooves with one lug in between them. Then, subtract half of the width of each of the grooves. Take three or four measurements and use light but firm pressure on your calipers.

When measuring the depth of the grooves, use the caliper's depth gauge or, better yet, a depth micrometer.

The reason all of this may be important to shooters is many of us cannot afford a high end scope for each of our rifles. Once throw-lever bases or other mounts are set to a particular receiver or rail, we might want to mount them on another railed upper for a trip to the range. It can be very frustrating to have to set the rings on your scope for the first rifle, and then to reset them again to put them on a second gun.

Another issue I have encountered is the depth of the cross slots cut too shallow, causing the cross lugs or bolts on the underside of the mount to bottom out without gaining secure attachment to the 45-degree-angle bearing surfaces on the rail. Accuracy will suffer and things can break, especially in the higher-recoiling chamberings.

Staying with higher-end AR receivers and rails will generally get you a true mil-spec rail platform for mounting optics or other accessories, but it is always a good idea to check, especially if you are spending your hard-earned money on a product that claims to be mil-spec.

 

Latest

Mossberg 500 and 590 shotguns
Mossberg 500 and 590 shotguns

Mossberg 500 and 590: America’s Defensive Shotguns

Since 1961, the O.F. (Oscar Frederick) Mossberg company has sold more than 11 million of its Model 500 pump-action shotguns, making it the most popular shotgun of all time, if not one of the most sold guns in any category, period.

Customizing the Colt Detective Special

Got a gun with that has seen better days? Perhaps Grandpa’s favorite gun was obviously “well loved?” Talented gunsmiths and other artisans are out there who can give your favorite firearm a much-needed face-lift.

First Look: Dead Air Armament Primal Suppressor

Dead Air Armament is adding the Primal, a new.46-caliber magnum rated suppressor to their lineup of firearms sound suppressors.

9/11 20 Years Later: A Special Smith & Wesson

There are still heroes in this world. We mourn the loss of one some 20 years later on the 20th anniversary of the September 11th attacks.

Why Defensive Firearms Training is So Important

Yes, you may never have to fire your handgun in defense of your life or family, but the possibility always exists.

Review: Smith & Wesson Shield Plus

In retrospect, Smith & Wesson had nobody to blame for the situation but themselves. The company didn’t invent the subcompact, lightweight, single-stack nine, of course. Walther and Beretta had preceded the original Shield to market by a few years with the PPS and the Nano, respectively, and Kahr had more or less created the niche back in the 1990s.

Interests



Get the best of Shooting Illustrated delivered to your inbox.