A firearm accessory mounting system includes a mount for attachment to a firearm mounting structure such as a handguard. The firearm mounting structure includes a slot. The mount includes projections around an opening for a fastener. The projections may, for example, be a slotted recoil lug. The opening includes first and second regions having different slopes. The fastener has a head which, during tightening, initially engages the second region to deform the projections radially outward against an end of the slot in the firearm mounting structure. Continued tightening of the fastener causes the head to engage the first region of the opening to apply an axial clamping force on the mount against the firearm mounting structure. This results in securing forces both transverse and parallel to the rotational axis of the fastener.