Building Fire KAC M4 Sliding Buttstock Mount

I love the look of a LMT SOPMOD stock. The sloping cheek rest is very comfortable, especially when I shoot through optics that are lower 1/3 cowitness on AR flat-top receivers. I just don’t find them quite as practical as some other stocks out there with sloping chest welds due to a lack of features.

One of these features I like on most of my stocks is a QD sling swivel mount, as I run most of my 2-point slings with a QD swivel on the rear and a snap/hook on the front. This allows me compatibility with all my slings with my AR platform carbines, as well as quick-detach, swapping, and re-installation of my sling systems, if I so choose. But what if you could mount QD swivels to your Gen 1 LMT SOPMOD (often referred to as “Crane”) stock?


I only recently stumbled upon this solution, even though the real and clone ones have been out for quite some time now – Building Fire KAC Push-Button Swivel & M4 Sliding Buttstock Mount (EP-64). More on the Building Fire KAC-clone sling mount after the break.

The buttstock mount is advertised as a “CNC Milled Aluminum” product (I only have a limited understanding of metallurgy and machining, so I can’t confirm this) and is finished in black with a KAC logo on one side. The included push-button QD sling swivel is steel and is finished in matte gray. The QD swivel can be mounted on either the right or the left side of the mount.


Pictured it here it is mounted to my VFC Crane SOPMOD stock in FDE (no markings). Quick note on these VFC “Crane” stocks – mine is actually quite nice and fits well on all of my MilSpec-sized Airsoft buffer tubes. In this picture it is mounted to my WE M4 buffer tube, which is indeed MilSpec sized to the best of my knowledge.


Two steel bolt clamp the two probably-aluminum halves of the buttstock mount. They are fastened in opposite directions, though the two halves and through the sling slot in your stock – this allows some forward or rearward wiggle room on where exactly you want your sling swivel on your stock. The mount is virtually wobble free when properly secured, thanks to the tension of the bolts and the geometry of the mount, which guides into place against the sloping cheek rests. Mounting the sling mount on the lower, vertically slot results in the tabs to release the buttplate being slightly pinched inwards (the same direction it takes to release the buttplate) on my VFC SOPMOD stock. As it is mounted right now, it is quite secure.


Thus far is compatible with all the push-button sling swivels I own and regularly use. Note that the sling swivels I own and use are a mixed bag, but all fit and function securely with Madbull, PTS, Ares, and Element sockets. I don’t own any of the shorter VFC QD swivels, so I haven’t tried with those. Rotation of the QD swivel is limited to 90 degrees due to the swivel knocking into the cheek rest at certain angles. I prefer the sling to rest at an angle that is nearly perpendicular to the bore when I let my M4 hang free, muzzle down… yet the angle at which the QD swivel is rotating to is perfect when I shoulder it, as it keeps any slack in my sling from getting caught underneath or around the buttstock.

It unfortunately does not fit without modification on my Magpul PTS MOE stock. I have not yet tried it on my CAR-15 or M4A1 LE stocks, though both of these are at my disposal – I never use them, so it didn’t occur to me until now to test fit the mount.

The real Knights Armament product, the Push-Button Swivel & M4 Sliding Buttstock Mount, is designed for older generation (pre-2010) LMT SOPMOD Stocks that don’t have an integrated QD swivel socket. It is also triple the cost of the fake ones, and seeing as how I’ve never seen one in person before, I figured I would just order a China-made clone prior to going all out on a real one. Chances are that the China-made clone one might even be good enough for my Airsoft purposes. There are other similar designs found in the real-steel world, from companies such as GG&G.

I purchased this from Asia with free shipping. I was not particularly impressed with the time it took to pack up and ship my order. It doesn’t help that they messed up my order and sent me some incorrect items and didn’t send me some items I did order. Asia Airsoft somehow is related to War Idiot, a smaller Hong Kong online store that I was very happy to order from, before their orders started to be handled by Asia Airsoft.

I’ve seen a similar product for sale on eHobby This one is advertised as a Big Dragon product rather than a Building Fire product though – however, they’re both low-end made-in-China brands with some sort of packaging to distinguish them, anyway. I’m skeptical as to whether or not they are in fact different products, though I see no reason to purchase another KAC-style Stock Sling Swivel Mount (at this time). Currently this “Building Fire” one is doing fine, though I haven’t gamed it yet to put it through some more intense field tests.



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