However, it proved less reliable at permanently destroying the Ferdinand heavy tank destroyer, whose bulkier, simplified design was more resistant to non-penetrating HE blast. While the Russians knocked out at least seven German Ferdinands in SU-152 ambushes at Kursk during one operation, German after-action engineers were able to repair, recrew and return nearly all to battle the next day. This has been attributed to the gun's reliance on blast rather than penetration, which disabled or killed the crew and damaged the vehicle's interior via concussion and spalling without harming the ammunition supply or chassis. In response, Soviet doctrine was changed by ordering SU-152 crews to continue firing on incapacitated vehicles until the turret was knocked off. After Kursk, the 152 mm BR-540 solid-core AP round was produced in small numbers and issued to heavy tank destroyer battalions in an effort to introduce a penetrating projectile, but the gun's inherent low velocity made the AP round no more accurate and only moderately more effective than the standard HE round (which could also be used against infantry).