Step 1: ensure artillery shell is defused
Step 2: apply ketchup
Step 3: massage ketchup across the entire surface of the artillery shell for uniform coverage
Step 4: heat lentil soup over medium heat for 10 minutes
Step 5: remove ketchup from artillery shell, rinse with water, polish
Fully vegan, takes 10 minutes
https://en.wikipedia.org/wiki/Canon_d%27Infanterie_de_37_mod%C3%A8le_1916_TRP
They were originally loaded with 30g of TNT. It was the smallest shell used in field guns, but meant to fire into confined spaces like bunkers where the pressure damage is amplified. The stereotypical WW2 American hand grenade, the Mk 2, had about 50g of TNT.
So what happens if you put the shell into a flare gun?
I should get a flare gun but don't have one. This is 17cm long and 4cm wide so I doubt it'd fit. All the internal components of the shell were removed by the artist engraving it so striking it wouldn't do anything mechanically.