Agree with Worra.
I'd keep the gun and get rid of your dad if he's that much of a bully??? You say he's kind, you say he's a bully, will the gun make a difference one way or the other to him being the man you want him to be?
Really beef up security, new front door too, one that doesn't need locking manually when it closes.
I'm not in Northern Ireland, I realise things are different there and will be for a long time even with the ceasefire, the old school will always be twitchy.
If no one knows the gun is there NOW, then I don't think there's a huge risk regarding intruders wishing to steal it, if its locked in a cabinet that can't be seen from simply peering in (or walking into the house) but he'd need to keep his counsell on that wouldn't he?
My neighbour had five shotguns robbed about 4 weeks ago. It was travellers (I am NOT saying they all are like this, but every year one team targets this area..) but we couldn't understand why it wasn't luck... no one broke into our house, or any other house on the lane, how did they know that the guns were upstairs locked in a cabinet in the spare room, and how did they know which ones to nick in the 2 minutes it took to start and finish the job?
They knew because my neighbour had been chatting to his mate at the bar in the local pub and so the theives knew exactly where to find them and which to take.
Idle tongues and all that.
I'm not sure you issues will be resolved if the gun is removed, I'm not sure your Dad will even see that as an option. So, compromise. Up security.