<p>A gun barrel wear is a critical factor affecting the performance and safety of small-caliber firearms. This study examines the failure mechanisms associated with barrel wear, focusing on the degradation of the barrel’s interior profile and its effect on maximum chamber pressure. The principal objective is to develop a formula that quantifies the effect of wear volume on maximum pressure using the double multiple nonlinear regression (DMNR) method. To this end, an experimental campaign was conducted, supported by numerical and statistical analyses. A Zeiss Micura coordinate measuring machine (CMM) with a VAST XTR gold sensor was used to scan the interior surfaces of barrels and combustion chambers at various stages of barrel life. High-pressure transducers measured chamber pressure, while light screens recorded projectile velocity. These experimental results validated a finite element analysis model, which was then used to create a database of maximum pressure variations throughout the barrel’s service life. A statistical analysis using the DMNR method was performed on the simulation data. This analysis produced a new mathematical formula that correlates wear volume with maximum pressure in small-caliber firearms. The formula shows a high correlation with experimental data, offering valuable insights into the relationship between barrel wear and interior ballistic failure.</p>