It's been 1972 since I cleaned an M79, but 40mm is 1.574798" and with the brush being a little oversize, I would guess that would work well.
With hot soapy water cleaning doesn't take much energy.
But between rounds one wants to eliminate sparks, and a wet/damp sponging is a great idea because it both cleans and kills sparks.
With the 155mm howitzers we had a 1' cube of a sponge to wipe the chamber and obdurating disks between rounds. (Dipped into a bucket of water each time.)