Hex Key

A hex key is a simple bent bar with a six-sided cross section that drops into the matching socket of a cap screw. Nearly every fastener on a modern precision rifle, from the optic mount down to the rail hardware, uses a hex socket rather than a slotted or Phillips head, because the six flats let you apply solid torque without the bit camming out and chewing up the screw. Both inch and metric sizes appear on guns, so most shooters keep a full set on hand.

The long arm of the L gives reach and the short arm gives mechanical advantage, which is enough for routine tightening of scope-rings, a scope-base, or m-lok accessories. The limitation is that a hex key gives no feel for how tight is correct, so for anything that affects zero or bedding it is better to do final tightening with a torque-driver set to the proper value.