Damn, I'm sorry this happened! You definitely need to talk to your dad and he needs to get himself an attorney. Withholding evidence is generally frowned upon. He should have turned it over to the police immediately. Plus he clearly needs to learn something about firearm storage and handling. ALWAYS proceed as though a found weapon is loaded until proven otherwise. Granted, you had no idea the the box contained a gun until it was too late. Dad could have at least warned you. How he "forgot" he had a loaded firearm in storage is beyond me. Now he can pay for the out of pocket portion of your medical bill.

Congrats on creating your own unorthodox version of Russian roulette

This sounds really fishy. A gun is highly unlikely to go off when dropped, especially a newer one. And if your pops was meaning to hand it over to the cops, why stick it in the attic instead of going right away?

I’m so sorry that happened to you. I guess this is a lesson for everyone to treat each box as though it has a loaded weapon for a just in case?