You are not the first person to burn a bridge and Lord knows, you will not be the last. Be the better man, suck it up, go back to the store when it is not very busy and ask for a moment of his time. Apologize about losing your cool, tell him how much you were looking forward to shooting the gun and how devastating the delay was.
If the gent is a decent human being, he will accept your apology and as a fellow gun buff, he will make it easy for you. Everyone screws up but few have the character to own up and make it right. The ball is now in your court.
The only thing I have to add is an old quote from Oscar Wild, "The two greatest tragedies in life are not getting what you want, and then getting it."
IMHO: Nothing can ever equal the joy of getting your first gun. (With the possible exception of your first high end gun - it's like acquiring a work of Art except even better because you get to take it to the range/field and shoot it)