When your son tells you his friend has a better scope on his Nerf gun than he does...
I happened to have the old crossman scope from my childhood bb gun (I'm kind of a pack rat). So with a little work in fusion 360, we made an adapter. If you have a different diameter scope or wish to modify the design, you can find the fusion 360 model here: https://a360.co/2yvLdVw
You may need to file down the locking nub to make it click into place and come off.