When Flying--Can I declare my firearm and put the locked case in my checked bag?

What the title says. I've flown with my firearm before, but only checked the case by itself. This time, however, I am planning to check a bag already. I want to use the original hard-sided case (lockable) that came with my gun this time, and would like to avoid paying for two checked bags. I am flying with United, and their website isn't clear on this. I am aware that firearms will have to go to baggage services, so they will have to add the tag on my checked bag. Also, I apologize if I used the wrong flair. via /r/CCW https://ift.tt/3st2RUD