ABSTRACT Industry has generally assumed that perforator penetration depth is independent of wellbore pressure. However, recent test data clearly show that modern perforator penetration performance is significantly affected by wellbore pressure. Two series of tests were performed using unstressed and stressed Berea sandstone. The unstressed tests used modified API Section II Berea test targets with wellbore pressure equal to pore pressure. Three different perforators at a 0.75-in. gun-to-casing clearance were tested. The penetration reduction from ambient to 15,000 psi wellbore/pore pressure was charge dependent and ranged from 22% to 28%. Tests with nonequal wellbore and pore pressures in unstressed Berea targets showed penetration reduction to be independent of pore pressure. Test of a single perforator in stressed Berea at an effective stress of 2000 psi also showed penetration reduction with wellbore pressure. This penetration reduction suggests that for the perforator tested, wellbore pressure may be a more dominant effect than in situ stress for deep wells.