In the world my friend and I are making, the different races throughout the planet have decided to work on advancing the development of Airgun technology. The main reason for this was that once gunpowder was discovered, it wasn't considered a very safe or practical gun-propellant since the oxygen in the planet was around 33%. Some individuals do still use gunpowder weapons, such as Blunderbusses and Sniper Rifles, but their commonality is seldom, since they are all muzzle and take a while to reload. The only benefit they do provide is Anti-armour and Anti-Megafauna use.
Airfiles on the other hand have a significant amount of benefits, since more time was spent on their development. There are is a wide variation of the gardidoni series, to Volcanics, Revolvers, Bolt-action Rifles; there are even Semi-automatic pistols.
However, despite their wide scope, one significant drawback is low range and power. Since the projectiles are too slow to puncture through armor and gambeson, a new type of ammunition had to be developed.
Instead of metal balls, the Bullets are made of an encasing of glass with a chemical explosive in the center. The idea is that glass itself would only break once it experiences a high force impact; typically subsonic. Once the glass breaks, the explosive is released, creating a high degree of heat so hot it can disintegrate flesh -- also leaving behind bits of glass that can act as shrapnel. Rather then relying on Puncturing the bullets instead rely on shockwaves to disable the target.
The problem that I've been trying to figure out is what substance can these people use as the chemical explosive that make these rounds effective?
An Idea I did think about was the use of fluorine gas. But the problem with that concept is how are they able to store it in the glass? The best idea we got so far was that instead of one chemical substance, there could be two. Perhaps once the bullet breaks, they could mix together and trigger the chemical reaction, but still, how could they put it inside the glass? And what two chemicals can work? I tried thinking about using Hydroquinone and Hydrogen peroxide(What Bombardier beetles use). But even then, I don't think it might produce the desired effect.
Also for specifics. The Muzzle of these weapons are about 11.7mm. While There understanding of alchemy is relatively advanced. The advancement in the technology is around close to renaissance era, with some advancements sprinkled in other areas. I could use a fictional material, but still. Are there real world chemicals we can use as some inspirational basis?