>>63953940
>>63954214
>>63954389
>>63954462
>>63954798
>>63956602
I'll let you in on a little secret.
This only applies to the plastic orion flare shells from canadian tire (dont try this with 12 gauge 2-3/4 shotgun flares), but the plastic body of the shell itself is the pressure vessel and barrel. The flimsy ABS flare gun doesn't hold any pressure.
You could use a cardboard toilet paper roll for a flare gun barrel and it would probably work with these shells.
Zamac would be overkill.