• Preset bubbles on projectile trajectory induce secondary water-entry impact loads, altering penetration dynamics.
• Rarefaction waves from bubble surface attenuate initial impact pressure peak by 68.8% and total specific impulse by 48.6%.
• Larger bubbles and higher projectile velocities enhance global deformation attenuation (over 80%) but reduce local rear-wall protection.
• Compressibility of bubbles is key to mitigating HRAM loads, but trade-offs exist for rear-wall perforation severity.