Think just about every reenactor or history buff has taken their hand in the kitchen to see what they can produce. Hoards of various "recipes" for them out there, some close, some not so much. Most appear similar, yet the consistency can vary greatly from cookie crumbles to rock hard hockey pucks that a ball-peen hammer upon it over an anvil couldnt crack.
Years ago some of my history pards sat out to analyze and create the accurate and authentic hardtack. Its what happens when you gather some history folks mustered up with science nerds, Chemists, and professional bakers. They were able to conduct chemical analysis on a fragment of a surviving piece of hardtack, then the race was on. In anything baked goods related the type and composition of the flour in question is the make or break factor. The Gluten levels within was one of the primary key factors. The usual array of grocery store flour types available today ie: all purpose, general purpose, self-rising, cake mix flours, etc werent even remotely close to a match. Hence the end product result was either crumbly, or rock hard. Nothing made today was an exact match but the closest found was between what is today called "Pastry Flour" and "Cracker Flour". Usually only found in specialty stores. They experimented with those and got fairly favorable results in both appearance and consistency. Taking it a step further it was quickly noted that the baking temperature was also a factor. Next batch they used the bakers commercial oven since it could easily reach and properly maintain the desired higher than average temps. Poof it worked. It accurately matched the original in all regards. For longevity it is also very important that all moisture allowed to properly evaporate out of the cracker to prevent any mold from forming. Just because it seemed totally dried out many times discovering soon that it actually wasn't.