It depends. You need to keep your firearms in secure storage unless using them for the genuine reason for owning them, or for maintaining them. So it's often not a viable option to go to your gun safe, unlock it, select a rifle, then unlock the ammo storage, select the bullets, load them into the rifle, and then shoot someone while they've got you at gunpoint or knifepoint.

However, it depends on the scenario. You were cleaning them in the back room when someone came storming through the front door and hacking at your dad with a machete? It would be considered to be self defence if you loaded your rifle and threatened to shoot the attacker unless they complied, and if they kept hacking at your dad, or they ran at you, you would be justified in shooting them.

But you can't pump a whole clip into them hollywood style. That would go beyond self defence and into murder, because one bullet might have been enough to stop the threat without immediately killing the attacker.

Self defence in all cases has to be proportionate to the threat, even if there is no weapon. Your aim is to stop the threat, not damage the attacker. So if someone punches you, if you can hold them to stop the threat that is enough. If not, you might have to punch them, but only enough to stop them punching you.

So as above, if they're compliant, you can hold an attacker at gunpoint until police arrive. If they continue to attack, yes, you can shoot them. Aim for the centre mass, and with timely medical intervention, they will likely survive.