Riding The Rails

posted on November 3, 2015

Remember quad-rail fore-ends? Those clunky, rail-laden behemoths are gradually being relegated to the antiquities department, but not long ago they filled a long-standing void: a secure and convenient way to mount rifle accessories. More flexible designs are all the rage today, with several customizable accessory-attachment systems instead of one replacement method for all items. What’s really awesome is the ability to mount only what is needed and where it is desired. The problem, however, is having different, incompatible methods for attaching rail sections and accessories. Nonetheless, customizable rail systems have taken the market by storm. I think it is safe (for at least another day) to break them down into four main groups: backer-plate, screw-on, KeyMod and M-LOK systems. Let’s take a look at each.


While it may look like accessories and rails bolt directly to the fore-end, backer-plate rails require additional hardware to ensure secure attachment.
Screw-on rail systems are easier to use than backer-plate systems, because the nut into which the rail section screws is already mounted in the handguard.

Rail sections that use a nut of some type inside the fore-end to secure the attaching screws predate the other methods of attachment. Back in the days when Picatinny was simply the hard-to-spell name of a military arsenal, specialized military units and law enforcement tactical teams were finding ways to attach critical accessories like gun lights and early IR aiming devices to their rifles. Electrical tape, hose clamps, flex-ties and epoxies were used with varying degrees of success, but things really started humming when some intrepid armorer decided to attach a cut-up Weaver rail section to a CAR-15 handguard with screws and hex nuts. Similar modifications to freefloat fore-ends were not far behind, though it took years for the marketplace to provide similar off-the-shelf solutions.

Backer-plate systems that utilize slide nuts on a track make for easier attachment than previous generations, which required the user to remove the fore-end to attach rail sections.

The modern versions of these rail sections typically attach via backer plates with multiple threaded holes in place of individual nuts. Some use slide nuts that ride in tracks inside a fore-end, making installation a snap. Design variations range from simple-to-attach to maddeningly difficult to get even a single screw in place. Once secured, backer-plate rails and accessories are solid. 

They are easiest to install when a handguard is not attached to the rifle. Once the tube is on the gun, you have to get the backer-plate (or nuts) inside—working around the barrel—and the further into the fore-end you need to go, the harder it is to secure everything. One brand used to come with popsicle sticks and double-sided tape to help place backer nuts inside the fore-end. While that worked, modern incarnations are much easier to work with. 


Not only is the screw-on method simple and fast, it also allows accessories to be quickly changed with the handguard on the rifle.
Two drawbacks to the screw-on rails are the wide variety of screw sizes and the need for caution when attaching steel screws to aluminum rails.

As the name implies, these attachments are mated by screws that thread directly into the wall of the freefloat tube or handguard. They have been around for a few years and are usually specific to a particular brand of handguard. Screw sizes vary, from tiny No. 4 to beefy No. 10 screws. Screw-ons are simple to attach—just align the rail section’s holes with the drilled and tapped holes on the fore-end, ensure the screw and hole threads are clean and then screw the rail section or accessory in place.

The end result of any good mounting system is solid attachment of an accessory to the rifle, but screw-on systems take a lot of the work out of getting to that point.

While premium fore-ends often have threaded steel inserts, most threaded holes are simply extruded aluminum. Take care that the steel screws do not cross-thread or strip the softer aluminum. One weakness with both screw-on and backer-plate systems is screws can vibrate loose over time. I recommend using a thread-locking compound for any rail sections you do not intend or want to move around. If your fore-end has a custom finish, go easy on the thread-locking compound. Using too much will cause the epoxy to seep in between the rail section and the fore-end, pulling the finish off should you remove the rail later.

I have also seen the smaller No. 4 attaching screws shear when heavy accessories are attached to hard-recoiling guns. The best bolt-on rails have shear lugs that index in slots milled into the handguard to provide additional strength when items are mounted on heavy-recoiling rifles. Overall, this style of attachment works well, and if every manufacturer used the same hole pattern and screw size, I would end my overview here.


KeyMod rails are instantly recognizable from the distinct shape of the attachment points. The “keys” are universal-fit attachments for open-source accessories and rail sections.
An oversize lip, which mates with the skinny section of the KeyMod hole, provides for secure attachment of the accessory or rail section.

These cleverly designed add-ons lock into a fore-end’s keyhole-shaped slots by way of screws and small nuts. The hardware varies in design depending on the manufacturer, but they all have a lip on one end. The rounded nuts are placed into the large hole at one end of the key slot, and then the entire rail section slides fully forward in the slot. The process is similar to that used when assembling metal shelving units, minus the pinched skin, blood loss and screaming. It does help to unscrew the nuts to the point where they are about to fall off the screws before attempting to insert them in the key slots. Once fully forward in the slot—which works with recoil to keep most things seated—the screws are tightened into the nuts, bringing the  larger diameter lips into contact with the inside of the key slot to tighten the rail in position.

One significant advantage to the KeyMod is accessories can mount directly to the rail, reducing complexity of installation.

The beauty of the KeyMod design is it standardizes rail-section attachment. Theoretically, one company’s KeyMod-based accessories should fit on another’s fore-ends. I have not had occasion to try every combination, but so far, so good on compatibility. The small nuts on the backside of each rail section have enough adjustment to account for different handguard thicknesses. Nevertheless, beware when loosening the screws—some nuts are not captive and once off, will bounce and roll to the farthest extent possible under distant furniture. Some shooters using large-caliber, bipod-supported rifles have reported needing to periodically retighten KeyMod mounts holding bipods on their guns. It has not happened to me with any KeyMod accessories, but at least one such report comes from a firearm instructor who sees a lot more KeyMod products shot through demanding courses of fire than I.

A nice KeyMod feature is in many cases, no rail is needed for specific mounts. Once these middlemen are eliminated, accessories can be attached using their own, built-in KeyMod nuts. That means less cost, less weight and less bulk. For instance, Primary Weapon Systems makes a bipod mount that has the KeyMod nuts on one side and a simple bipod stud on the other. KeyMod light mounts, vertical grips and sling attachments are also offered by various companies. 


Not only are M-LOK rails easy to use, simple-but-sturdy accessory attachment can be achieved anywhere in the slotted sections.
Self-tightening nuts turn 90 degrees to lock the M-LOK accessory or rail section in place. Like the KeyMod system, M-LOK accessories can mount directly to the fore-end without the need for a Picatinny rail section.

Earlier this year, I received a few Geissele Automatics MK8 freefloat tubes with an all-new accessory attachment system known as M-LOK. That “M” comes from Magpul, the inventor of this locking system. The M-LOK rail sections made by Geissele are as bombproof as anything I’ve ever seen. I have since ordered other fore-ends with M-LOK slots and relied on them heavily for numerous rifles.

This attachment method is similar to KeyMod in that rimmed locking nuts slip into mounting slots on the rifle’s fore-end. But unlike KeyMod, you don’t have to fight to keep the nuts from free-spinning as you try to tighten them. Once you have the rail or accessory fully forward in the mounting slot, tightening the screws causes the M-LOK’s T-nuts to rotate 90 degrees. At this point they cannot spin any further. Continuing to tighten the screws draws the T-nuts up against the underside of the freefloat tube or fore-end. The process takes only a few seconds and yields a very strong attaching point for numerous accessories.

M-LOK may be the simplest attachment system, but accessories are nonetheless securely mounted to the rifle.

Magpul offers different lengths of polymer and aluminum Mil-Std 1913 rail and cantilever light mounts for freefloat tubes and two-piece handguards. Both KeyMod and M-LOK attachments have shear lugs on them that both provide additional linear strength and serve to hold rails and accessories in position in their slots. My one complaint about the M-LOK system is simply it creates another dang option I have to contend with, but since I regularly make the case for modularity and having more options rather than fewer, I really cannot gripe too much about this new option from Magpul.

Interestingly, Magpul is offering a free license to use the M-LOK system to any manufacturer, provided the M-LOK logo is used on the product. Therefore, we should see a host of accessories specific to M-LOK before too long, given the system’s utility and Magpul’s market penetration.

Beyond Tomorrow
My crystal ball does not tell me if the next ingenious accessory-attaching system is right around the next corner or 50 years distant. If I were to hedge my bets, I would predict that before long someone will come up with yet another option for sticking things to guns. So long as it does not add bulk or weight, require a mastery of physics to install and is not exorbitantly priced, I will celebrate.


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