If that is the case, I think it's a bad practice. During an actual engagement, there will not be any time to set up nets and mats. It is during these practice runs that any issues with shells leaving scratches or gorges on the deck should be discovered and learned. This type of feedback can help improve operational process as well as lessons learned for future designs.
During an actual egagement, no one gives a damn about scratches and dents. Also, literally every navy in the world does this; lay down protective mats during live-fire practice of the main gun. Not a single navy has thought it worth the money to replace steel with an exotic non-scratch, non-dent material around the main gun.