If this lug doesn't do that you do not have your accessory installed correctly. These lugs contact that larger hole in the KeyMod slot and make it so the accessory can not slide backwards. In addition to the shoes and screws for tension every KeyMod accessory piece I have had my hand on has had some sort of lug that rests in the back of the large slot between the two screws or forward of the front tension screw.

This can cause slipping and wiggle also and is hard to see due to how tight most of these handguards fit to the barrel. If you allow your shoe to be too loose before fitting it into the handguard you are using it can become crooked and even though it feels tight as far as torque wise it is just resting on the edge of the slot it fits into and not doing a thing for your tension. Like I said earlier the shoe should fit snugly down into the slot, this fit combined with tension is what serves as a backer. This has been the issue with several of the QD cup sling mounts I have seen in regards to improper installation. This causes an issue with your accessory not locking in correctly and having the ability to slide loose. If is in backwards like the photo below it causes a slight space between the edge of the shoe and the end of the KeyMod slot, which you can see in the next photo. There should be no gap or space between these pieces. This allows for the backer nut, which I am going to refer to as the shoe because that is exactly what it looks like, to slide in then forward as far as possible so that the shoe rests in the machined slot that it is made to fit into. The photo below shows the proper orientation for the backer nut in relation to the KeyMod slot.

KeyMod is obviously a larger hole that is connected to a smaller, longer slot with a half circle on the muzzle end.

Please do not think that because I am doing a write up that KeyMod is complicated, because it is not, in fact it is probably simple enough that some folks are over thinking it. so I thought I would throw together a write up with some photos. Alright folks, while bouncing around the forums I have seen a lot of people who either were against KeyMod, didn't understand it or loved it but were having issues attaching accessories.

Keymod accessories install