Below are the drawings extracted from MIL-STD-1913 I have redrawn them in AutoCad and converted to jpg for convenience. I know some have had problems with the fit of various components to their supposed MIL SPEC rails. Here's the MIL SPEC dimensions for anyone who wants to verify their components to MIL SPEC. If anyone doesn't understand the geometric tolerancing symbols etc., let me know and I will explain it to you.
--RR

FIGURE 1

FIGURE 2

Whoa, nice.  I've wondered about the dimensions of the 1913 rail specs.  Dang it, now I'm going to have to stay up half the night micrometering my AR.  [;D]

Ghost

The drawing is real, however the average macine shop have trouble measuring these dovetails. First of all, the .835 dimention is not a critical dimention as signified by the fact that nothing attaches to it. Everthing to be attached has to fit the angles, not the blunted points. Each of the top angles must be messured to the coresponding angle on the bottom opposite side, much like forming an X.
Then you have to be able to measure the parrallel angles to afirm that they are seperated in width to the spec. RRA are not the only ones to get it wrong, so the average guy has no chance unless he has all the right inspection room tools, and then knows how to realy use them.
God luck, Jack  

It's interesting to me that my ARMS #38 only measures .285" for the total height (speced at .367" minimum).  I guess that explains why once or twice I've run across things that would not fit the ARMS rail.  I always assumed they were out of spec, not the rail.

Yes, the drawing is definitely weak though the dimensions can be checked. At my company we check our NVD mounts using a CMM but if I didn't have that option I would design 2 sets of go/no go gages to verify the "functional" attributes which are the .748 (datum C) and .164 dimensions. Those are the surfaces most likely to create interference with a mating part if out of spec. Seems odd companies like RRA and ARMS apparently don't do a simple go/no go check. Such a "no brainer" check would at least raise a flag that there is interference and the rail attributes need more comprehensive inspection --RR

The ARMS rails are known as the most accurate in the industry and are std. to spec. and been passing gov't inspection for 20 years. So something measured might dif. in the way it was done. I have never seen anything from  ARMS out of a spec that is meant for military. That would be kind of hard since the Picatinny 1913 rail derived from the ARMS rails developed by ARMS in the mid 80's.
Good shootin, Jack

JohnM, new-arguy, Do you have any appreciation for the value of these drawings or do you understand this is the MIL STD 1913??? Get a fucking clue and pin the drawings to this forum or the general ar15 forum
--RR

I have heard the Tango Down front grip will not fit ARMS rails because they are not to the 1913 spec.  Are you saying the Tango Down grip is whats not to spec 3rdtk?

I certainly hope these drawings are used by all for individual purposes. I'm the one who obtained the Mil-Spec from Picatinny Arsenal and sent it to Andy to post on Bighammer.  

That was 2 years ago and there was no link here on ar15.com to attach it to at that time. Those that visited Andy's web site have known of the Spec for quite some time. Sorry it's come to light only lately.


JohnM in Iraq

I don't know anything about that copy of the Spec or drawings. I created the drawings (above) with AutoCad from my own copy of the Spec... i.e., I work for a defense contractor and we work with this among many other MIL STDs. The reason I redrew this is the original is a PDF file and is very poor quality. I can post the entire verbage of the Spec but these drawings are one needs to verify physical conformance. --RR

3rdtk

This is how I read the GD&T.  Please advise.

The Geometric Dimensioning and Tolerancing (GD&T) callout uses the M modifier.  This specifies perfect form at MMC or max material condition.  The tolerancing calls for .006 from the C datum at MMC using a TOP (Tolerance of Position Modifier).  TOP for an external FOS (Feature Of Size) equals MMC (.835) plus the TOP tolerance value. (.006)  This tolerance is applied to the virtual condition boundary 90 degrees to datum plane C and centered between the basic dimension.  The part will look like this:

The width of the rail is .835 - .005. MMC is .835 and LMC is .830 with TOP callout allowing for .006 float centered 90 degrees from the C datum at MMC centered between the basic dimension.  This arrangement would yield a bonus tolerance of .006 at MMC.  

The rail can be a maximum of .841.

The way I see it the .835 dimension is critical.  It is the dimension that sets the position of your angles.

The (theory) is correct in the placement (off) the points when setting up to machine. However, the approach to get to the dimensions vary from manufacturer to manufacturer for some very good reasons. The gov't inspections do not care how you get to the correct dim. only that the measure is correct to the spec.  You are very correct about how this print makes the points prior to the clearance cut important. Manufacturers have found that this particular print and datums approach is not as good as others used, due to the fact that it makes the points as important as the angles, and that shouldn't be. The angles are important (critical)part. The theory of the points is hard to control due to the severe way the sharp alum. is hard to control, to a very fine edge, much like a knife, prone to an accidental ding that messes up measuring with a dial indicator and/or on a comparitor, and just not as accurate and or controllable on a production line. Handling these sharp edges without damage to the point, measuring, cut hands, and a general pain in thier assets. Yes, when in a CNC machine the rail will have the points machined off, but the other problems do creep in. The thickness of the plating is another factor that can also cause variations, but that story some other day.
I have seen various proprietary methods, and the results are good from the mil spec makers.
 If you examine all the various attachment devices that have been used for many years, such as the Leupold rings, KAC attachments, ARMS mounts, various mounts from the NV and thurmal makers such as Raytheon, Litton, the former Hughes Corp., PEQ laser attachments from Insight, etc., they all have a maximum dia. that also fit all the dovetail mil-spec rails to include the ARMS rail, which were actually the earliest 1913 rails. If a certain new pistol grip doesn't fit, does that mean all the other long time experienced makers are wrong, I don't think so. I'd suggest that the new pistol grip maker (any new maker) take a second look at what has been and is in service out there, and correct his mold to be in standards, set up years ago.
Good shootin, Jack

I agree with your assessment.  I think the GD&T and the overall dimensioning sucks.  It would be clearer to use a profile tolerance.  Do people who make rails use a full form GO/NO GO gage or do they use some type of variable gage to measure on the production line?  I think the datum scheme sucks as well.