What is this coil going to do in the topper? Is it going to be a complete pop bumper assembly or a knocker or a bell?

Be cool when you add a little red light in the end of the barrel to simulate a muzzle flash each time your coil is fired.

Green - You could probably put a red 48v panel indicator (pilot light) in there and just wire it directly to your coil. You wouldn't have to run separate 6v wiring or a switch for the lamp that way.
Something like this should work if it isn't too big for your barrel. It's a little over an inch in diameter.
https://www.amazon.com/Pilot-Light-Panel-Indicator-10pcs/dp/B01N68YLSV

Is the coil going to hit a wooden base and sound like a knocker or do you have a different sound in mind?

Also, Is your gun going to be pointed forward as if it was shooting at the player?