I bought one of these MI handguard rail sections with the intent of drilling new holes in it that would be in the appropriate places to allow it to be mounted atop a Ruger Mark II selfloading pistol, using the existing (drilled and tapped) holes in the Ruger's barrel/receiver assembly.  The idea was to then attach an Aimpoint Micro T-1 in a LaRue LT661 QD mount (the short one) to the MI rail.

After receiving the rail, I tried to attach the LaRue mount to it, but it would not fit!  The cross bar on the bottom of the LaRue mount is too wide to fit into the cross slots of the MI rail.



According to my digital caliper, relevant measurements are as follows:


MI rail section cross slot - 0.190

LaRue QD cross bar - 0.200

Picatinny spec cross slot - 0.206



Here is an engineering drawing with a callout indicating the proper width of 0.206 for the Picatinny cross slots:




I measured the cross slots of a CMT flattop upper, and they conform to the proper spec of 0.206.  Consequently, the LaRue QD mount fits perfectly atop the upper.

At issue is the fact that the LaRue mount and the MI rail are in conflict, since a cross bar with a width of 0.200 will not fit into a cross slot with a width of 0.190.


These MI rail sections may be appropriate for mounting lights and lasers, but they are NOT Picatinny spec.


I can machine a piece of rail of the proper dimensions, but prefer to purchase one that is made to the correct Picatinny spec.  Then all that would be necessary would be to drill the holes in the appropriate places to allow it to be mounted to the Mark II.




My question is as follows:

Can anyone provide a suggestion for the purchase of a rail section that has the proper 0.206 cross slots?

I bet if you made MI aware that their rail sections weren't within spec, they'd take care of it for you.

If you don't get anywhere with MI, you might also consider calling Vltor about purchasing some of the short rail sections they produce for their CASV handguards.  They come in 2, 4, and 6 inch sections.  I've been adding them to the bottoms of my bolt rifles for use with my 1913 bipod adapters.  I like them because they are completely flat on the bottom when compared to other rail section brands.  

Don't file the slot wider.  You'll never keep it square, you'll be removing the tough anodized coating, and it will look like shit whenever the QD mount is removed.

I'm sure they wouldn't care because its a Larue mount, and then they'd try selling you some ADM products.

Quoted: I'm sure they wouldn't care because its a Larue mount, and then they'd try selling you some ADM products.

Gimmie a break . . . out of spec is out of spec . . . end of story.

There are a few manufacturers who like to use "thick" recoil lugs on their mounts, not just LaRue.  Seekins Precision scope rings immediately to mind.  I've personally had this same problem using Seekins rings with another brand rail that had slightly small and out of spec slots.

What is interesting is that more than a few manufacturers seem to be misreading the 1913 spec.  Correctly interpreted, it is;

0.206"  (+0.008/-0.000)   and NOT  (+/- 0.008)

The drawing dimensions specify the minimum material condition, not the nominal condition.  And no, I'm not a machinist, but I did stay at a Holiday Inn Express last night.

I curious to know now if any MI railed handguards have this same condition?

Quoted: Depends on how talented you are. I could do it and you'd never notice.

You must mean, except for that silver color and file marks . . .

Quoted: Nope. No silver color, no file marks.   You underestimate my skill with a file and Alumablack.

Well at least you didn't propose using a Sharpie marker . . .

Quoted: Quoted: I'm sure they wouldn't care because its a Larue mount, and then they'd try selling you some ADM products. ????

www.ar15.com/forums/topic.html?b=2&f=219&t=163284

Quoted: Quoted: Nope. No silver color, no file marks.   You underestimate my skill with a file and Alumablack. Well at least you didn't propose using a Sharpie marker . . .

What's wrong with using a sharpie?  How about a Marks-A-Lot??