Allen screw 
noun trademark
A screw with a hexagonal socket in the head.Both blued and stainless versions are offered with an Allen screw in the head of the pin that can be tightened against the bottom of the barrel to prevent movement under recoil....
- There's also an Allen screw tension adjustment to regulate how tightly the pistol is held.
- The standard pin has a larger head complete with an Allen screw for locking in place.
Origin
1930s: from the name of the manufacturer (see Allen key).