The main objective of this paper is the development and validation of a three dimensional thermomechanically coupled finite element model for gun drilling AISI 4150 quenched and tempered steel. The Lagrangian formulation proposed in the FE code DEFORM 3DTM and the constitutive Johnson-Cook material model were utilized to simulate the chip formation and to predict the cutting reaction forces as well as the temperature in gun drilling process using carbide gun drills with two different diameters. During gun drilling simulation, modified gun drill cutting edge rounding and friction law are performed to investigate the effect of tool wear and lubrication on feed force and torque respectively. Experimental gun drilling tests were carried out in steel AISI 4150 for the validation of the developed 3D FE model. The developed and validated 3D FE model can be used for optimizing the cutting process in gun drilling (good surface finish and straightness) taking into account the complex gun drill geometry, cutting conditions, heat transfer and the thermo-mechanical behaviour of the workpiece material.