SpudFiles

So how does the grenade go off? The bottle splits in half. When the cap hit the ground it would shatter.

Um, what? The grenade detonates by blowing a hole out of a weak point in the bottle. The cap would be at the rear of the projectile, and would not hit the ground nor rupture on impact.
Either way, this deign could work as a mortar/an intense single shot shotgun.

I never experienced this, the cap would shatter long before the bottle would burst.

What were you using as a shell?
And how much vinegar/baking soda?

Oh who cares, it doesn't matter. After getting shot out of a combustion cannon the force of impact will make the bottle pop I'm quite certain.

Honestly shooting a bottle out of a a mortar towards people sounds like a bad idea. Id go pneumatic so you can control the power. And I was using a water bottle.