(I'm going to remeasure this later on to make sure I managed to remember the correct numbers long enough to type this.)
Last year's XCR-L MLOK handguard width: 1.52 inches
With Leapers Low Profile handguard covers: 1.68 inches
Norinco NHM90 original wood handguard sanded and stained: 1.52 inches
I have been trying to figure out how to make a AK style handguard cover for the XCR, with the bulge toward the receiver end (a more subtle angled handguard), and possibly the widened lower section that I think they call a "Polish Swell".
I'm not sure now if that's really needed. The 1.68 inches with the Leapers seems good.
The upper has three areas that are definitely rails: left, right, and bottom. There are four rows of slots that I question as being usable or decorative. The left and right 45 degree facing up beside the gas tube appear to be decorative (speed holes or ventilation) because the back sides do not...
I didn't follow up after Sean's response, but after reading his response and taking a good look, the lower diagonal slots (4:30 and 7:30 positions) do not have the same seat thickness, are too close to the barrel to fit the nut or any bolt sticking through the nut, and may be curved on the rear face, instead of flat. The upper diagonal slots (1:30 and 10:30 positions) are also have a thinner seat surface, and are reverse curved to go up over the gas assembly.
Conclusion was that they are decorative slots matching the appearance of the adjacent actual MLOK slots (3:00, 6:00, and 9:00 positions), and function only for air circulation and cooling of the barrel.
Been staring at that picture for about a month now...
OK, got out the rifle to change and locktite the little bolts of the rail covers from 3/8" to 1/4". Did not successfully find shorter bots for the sling mount.
Shot a picture of the calipers across the width of the rail. 1.5225" with my thumb holding the calipers too tight while pressing the camera button with my left hand. Would be 1.524" without the pressure on the thumb wheel of the calipers. This rifle was billed and shipped at the end of November of 2021. Ordered at the end of March of 2021.
That's a difference of .006", or the thickness of one and a half pieces of 20 pound copy paper.
After a month of staring at that picture, and those rifles were both being manufactured in 2022, I'm thinking that if there was a change, it was in the depth of the cut between the rail positions (1:30, 4:30, 7:30, and 10:30 positions), resulting in an optical illusion that the rail surface width changes...
...or that the different angle of the two rifles created different shadows, making it appear that the rail width is different.
MLOK was invented by Magpul. They do not show a patent on the rail, and there are notes saying the rail pattern is free use. Magpul licenses the use of the MLOK name. So it sounds like anyone can make a rail using the slot pattern, but must pay Magpul if they want to identify the pattern as MLOK.
Magpul does have patents on every possible thing you can attach to that rail, and the little nuts that hold those things onto the rail. And they have trademarked every name of everything. Patents run out in 22 years, and trademarks can be renewed for as long as they are in use. So that is a long-game approach to making money.
Most of the drawings for the rail show only the length and width of the slots, shape of the corners, and spacing between. Then the back side of whatever attaches and the little nuts. The only drawing I came across that has a section marked with the little arrows that point to the side and the letter "B", does not have the drawing of the section included.
There are drawings of rails from different makers of floating hand guards, and they ignore rail thickness or width of the hand guard (beyond one saying 1.5 inches), and seem most concerned with a spec for the air gap between the barrel and hand guard, and the depth of the slot for the gas assembly above the barrel.
The standard bolts for the MLOK rail covers, sling mounts, and Picatinny rails to mount to the MLOK rails seem to be 3/8 inch, and too long for the XCR. 5/16 would be better, but apparently not made in low socket head cap screws, and I wanted a fully coated bolt that would not rust on the end like if I sanded it with the belt sander. I ended up with quarter inch.
So, it appears that the XCR has thinner than standard rail cross sections, that leave more bolt sticking out than other rifles and hand guards. And a minimum clearance between the inside of the hand guard and the barrel.
Whatever the end of the hand guard width discussion, plan on buying a bunch of 1/4 inch long socket head cap screws to replace all the 3/8 inch long bolts that come with all of your MLOK accessories.