Two thoughts (like most at first blush I'd have guessed way high....).... I never guessed how tough a lightweight composite of metal and plastic could be( single keyboard). Secondly the sandwich effect (different layers spaced apart at regular intervals) provides a lot more ballistic resistance than expected (the keyboards although bound together by tape are actually very resistant to penetration -multiple keyboards).
Now, of course, we need to see a few different "penetrators" in the same media (a hot pistol round or two.... several rifle calibers (starting with 223, then 7.62x39, then 30.06...). Interesting results (and yes, boys don't grow up -they just get older....).