As an addendum to the writeup above... just how "hard" is hard-tack? Well, consider this amusing anecdote, dating back to the American Civil War, which about sums up the prevailing attitudes towards the fare. Preface this bit with the realisation that food rations during the war often arrived in bad shape, some having sat around in dark, damp places for many days or weeks.
First Soldier: You know what? Today I was eating my hard-tack when I bit into something soft!
Second Soldier: Well, what was it, a worm?
First Soldier:: No! It was a ten penny nail!