Doom's standard fist attack. This checks whether the player has the Berserk pack by looking for the presence of a PowerStrength item in the inventory. Damage is 2x 1d10, multiplied again by 10 when berserk.
You can implement A_Punch using A_CustomPunch. You can find an example of how to do this on that action's page (near the bottom).
This example is taken from Doom's Fist weapon.