You can see the smaller-than-bore firing chamber on this diagram of the Tannenberg hand gonne, mid to late 1300s. In one of my books on hand gonnes to matchlocks, I recall reading that if packed too tight, the early powders would not combust to explode efficiently - they would just fizzle.
Link to a cool article on the early gun powder recipes:
Synopsis - Co-authors Dawn Riegner and Clifford Rogers, a chemist and historian, respectively, at the US Military Academy (aka West Point), decided to analyze the energies released just before and during combustion by different kinds of gunpowder recipes from the medieval period. Along with their other co-authors, they hoped to better understand the intent behind the creation of the various formulations and to learn more about the technical details of early gunpowder manufacturing processes.
First, they identified more than 20 different recipes recorded in medieval texts from between 1336 and 1449 CE and followed those recipes to make their own different batches of gunpowder. Riegner et al. tested both serpentine and corned samples, using bomb calorimetry to record the relative heats of combustion and reaction rates. They used differential scanning calorimetry to measure the onset of combustion (preignition), and how quickly combustion spread, as well as analyzing the residues for each of the recipes to determine the effectiveness of combustion. The team also compared the different sample preparation methods and the effectiveness of recipes with and without additives, as well as conducting the firing-range cannon experiments.
The team found that, between 1338 and 1400 CE, recipes increased the percentage of saltpeter and decreased the amount of charcoal. This would have resulted in lower combustion heat but would also have been safer for medieval gunners on the battlefield. After 1400, gunners tweaked the relative components a bit more, decreasing the saltpeter a little and sightly increasing the sulfur and charcoal, perhaps to find the optimal balance between gunner safety and combustion heat.
https://arstechnica.com/science/202...der-recipes-with-15th-century-cannon-replica/
Conclusion - "It is clear that medieval master gunners had developed, at least in some respect, a solid practical understanding of the variables that affected the effective power output obtainable from gunpowder charges, including purity of ingredients, varieties of charcoal, grain size, and methods of mixing," the authors wrote. "They understood, for example, that a cannonball was thrown by gas pressure, not flame, and that willow charcoal prepared in a closed container was far superior to oak charcoal made in a traditional pit."
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