Autoaim

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Vertical autoaim

Since you can't look up or down in Doom or Doom II, the system compensates for you. Accuracy is not affected by the differences in the Z-axis co-ordinates of the player and target.

Horizontal autoaim

The player's projectile weapons have a limited ability to adjust their direction if the nearest target is not exactly straight ahead. From P_SpawnPlayerMissile in p_mobj.c:

   // see which target is to be aimed at
   an = source->angle;
   slope = P_AimLineAttack (source, an, 16*64*FRACUNIT);
   
   if (!linetarget)
   {
       an += 1<<26;
       slope = P_AimLineAttack (source, an, 16*64*FRACUNIT);
   
       if (!linetarget)
       {
           an -= 2<<26;
           slope = P_AimLineAttack (source, an, 16*64*FRACUNIT);
       }
   
       if (!linetarget)
       {
           an = source->angle;
           slope = 0;
       }
   }

An angle in the Doom engine is 32 bits, so that 1<<26 = 360° × (2²⁶ ÷ 2³²) ≈ 5.6°.