What kind of charcoal did you buy? There is a significant difference between wood types. The majority of charcoals you're going to get for cooking will be oak or mesquite which are exactly the opposite of what you need.
Charcoals all have energy values. The secret that is never spoken is that it all really relates to density. Cellulose really only differs in the sugar, water content and the amount of porosity.
This is where it gets interesting, anyone who is more than an amateur pyro-technician (I love what that entails) will tell you that BP is all about the charcoal. That is assuming the corning, granulation and water content are held constant.
Let me show you a couple of things. This is the densities of two charcoals good for BP and three common fuel charcoals. Note that the densities for "lump" charcoal as you are calling it is very low. The second column is for the milled charcoal. It has been rendered to the powder form necessary for creating BP.
Quite a range isn't there?
Now break that into a chart including Potassium Nitrate and Sulfur and we will be using the units for density of grams /cubic centimeter because all of the records I have are in those units.
In this case note that this is for milled KNO3 and Sulfur. It is the opposite of charcoal, it starts out much denser and is "fluffed" up by the milling.
Then a pressed cake is formed which will have a density ranging between 1.4 to 1.8 g/cm3 depending upon the charcoal type. After it goes through a breaking mill, the ball milling and barreling the densities will translate to this:
Notice that if you use weight for the 75/15/10 ratios you end up with a 254 grain difference in 1 pounds of yielded powder? Charcoals are different, and different manufacturers uses different charcoals and they yield different energy factors. You've probably seen chronograph from different powders and they vary.
The secret is the powder density. Lighter woods are better, porosity in the charcoal is better. and here is the real dirty secret...It's volume. If you mix by volume and abandon the 75/15/10 weight ratios you can get similar energy factors with different charcoals. The denser charcoals have lower energy factors but if you mix them by volume then you can regain some of the difference because you have added fuel to the mixture. Otherwise you have too much oxidizer and too little fuel by volume. Guess what? Combustion is volume driven.
Bet your amateur pyro-techs didn't tell you that did they? Commercial mills that are producing based on gas and energy generation tests vary their mixtures to yield consistent results with similar volumes. 75/15/10 is just a starting point. But if you track the mixtures a trend shows up and it is volume driven.
Well back to your bags of charcoal...What did you get and how will they mill it the required density?
Regards,
Mako