You could have a small stick like thingy attached to the guard of the gun in order to push the dart inside or maybe something better that is fitted into the gun
I know I'm A couple days late, but my unicorns safety broke/wore down, and I haven't used it for maybe a half a year, it seems my copy's spring was too strong and has warped it to the point I can't keep it at on!, it disappoints me as I was ready to bring it to battle