Passing by from America. I wish I spoke Sri Lankan because you seem cool bro the type of guy I’d let roll with my guys and make some mayhem lol. Take care friend!
A wadding of a 20x20mm piece of tissue paper, wrapped around the marble, and using a stick to push it down the barrel to seat it, would help increase the speed it will travel at. It would also hold the marble at the base of the barrel so it won’t fall out if you aim downwards. Don’t make it too tight of a fit in the barrel though, don’t want to burst the gun.