Increasing the bore volume will make it quieter not louder when using a
bullet.

Smokeless gunpowder is a propellant that burns, not an explosive.
What makes a bang is the pressure behind the bullet when it is uncorked.

If you are shooting blanks, smokeless powder has problems making noise.
Black powder is an explosive, but it rots the bore if not cleaned, and
who want an explosive around?

I have make big bangs out of rifles with no bullet and smokeless powder
when forming cases. I put 10 grains [.0015 pounds] of powder over the
primer and then 30 grains of Cream of Wheat cereal over that. That is
loud fired straight up, but can fall out if the barrel is tipped. I put
some Ivory soap over the Cream of Wheat to keep it in place.

As calculate on rec.crafts.metalworking for me, the threshold of guns
being loud is when the escapement gas is at least one atmosphere above
ambient.