Ferret, the plastic test cap is made of LDPE, so it's flexible and won't break from the impacts, and the shards of puck digging at it. My baseball bat with the square meat tenderizer hammer head mounted into the end of it is raised and brought down hard on the puck, inside of a 3" ID ABS pipe about 18" long. Confining the BP this way assures that each hit does work, and no chips fly out of the tube.
I've got it down to a repeatable process that gives very consistent results. The grade I use for baseball testing is 2FA, and I am personally surprised that my redneck 'corning equipment' gives me very close to 2/3 2FA every time. I give a few hard whacks to break the puck up, screen through 4 mesh, return what sits on the screen to the tube using a large funnel with a large mouth (the top of a water jug), repeat, repeat, repeat, etc... As the amount of BP getting crushed gets less and less, I use less muscle to break it up. Before I get to the last bit of a puck, I'm introducing the next one, so I'm not beating on a few grains and crushing them to a finer grade than I desire. For the grade you want, you'll probably use more muscle and less screenings. The more 'sessions' of crushing and screening, the more consistent the result.
What I do to get consistent density is to use a puck die that has a 'stop' on it. The sleeve is bronze 'oilite', and the 'plunger' is aluminum, made for me by Woody's. It has a head on it that stops the plunger at 1/2" puck thickness. It was made, taking into account the pressing base that goes into the bottom of the sleeve to press against. The base goes 1/4" up into the sleeve. Each puck is pressed to exactly 1/2" thickness this way- although, there is a slight amount of expansion when the pressure is released- maybe .002 or .003".
When I press single 2" diameter X 1/2" thick pucks, I use about 4 tons of force. The force is increased slowly, with some dwell between cranks on the hand-cranked hydraulic pump. Once the stop bottoms out on the sleeve, my puck is at the right thickness. It takes 3-5 minutes to make each puck. The density is controlled by the weight of powder used for each puck, with water considered as an additional percentage to that.
Kyle's use of 10% water is certainly excessive for MY application. It should be kept in mind that Kyle is pressing multiple pucks at one time for general (pyro) use, and that the large amount of water is being used to facilitate that. My process is for test pucks, and would lend itself to firearm use (I think), because it makes (potentially) more consistent powder.
I tried to press a puck using no water at all, just dry powder. I aborted after raising the force to 7 tons, almost double what I normally use. The resulting 'puck' crumbled upon ejection. The function of water in the production of black powder was shown to me in a very obvious way. Also, your observation that breaking up damp pucks makes more fines coincides with my experience.
I don't want to comment at this time on what percentage of water (I think) is ideal for single pucks until all my testing is done. Working on it now.