The erosion and wear in the gun barrel will get worse with an increase in the number of projectiles fired. Generally speaking, the ballistic performance of the gun, measured through indicators such as the maximum pressure and the muzzle velocity , will decrease gradually due to gun barrel erosion. However, the above analysis does not agree with the firing test data of certain types of guns, especially of some small-caliber guns. The ballistic performance of such guns will exhibit an increase to their peak values followed by a gradual decrease with the number of rounds fired. This is the so-called interior ballistic peak phenomenon, also named as the hump effect. Taking several kinds of guns as examples, such as a 76 mm gun and a 100 mm gun, we calculated the engraving pressure of the guns by an approximate method and built a lumped-parameter interior ballistic model of the guns that exhibits the effect, according to the interior ballistics theory of guns with erosion and wear. The results of the modeling of the guns under different wear conditions are close to the test data, showing the existence of the peak values of and . The simulation results of some of the other guns that exhibit this phenomenon also show good agreement. Furthermore, it can explain the double-peak phenomenon for some types of guns with double driving bands. It was proven that the mismatch of the structure and the dimensions of the gun bore with those of the projectile driving band is the fundamental cause of this effect. Due to the mismatch, the engraving pressure will first increase and then decrease with the enlargement of the bore dimensions caused by barrel erosion and wear. The variation in the engraving pressure will inevitably lead to the variation in interior ballistic performance in the life cycle of the gun. This observed process appears to explain the interior ballistic peak phenomenon.