Think the issue that OP will have re proving that it was "reasonable" to shoot 4 bullets through the door is his knowledge and experience with guns
I have never shot a gun in my life. So might think if I shoot just one bullet it might get stuck in the wood, not go through the door and be enough to frighten off the "burglar". But I've never shot or held a gun in my life. I might get a fright when I shot the first bullet and then as a reflex keep shooting the other three (note: I wondered when Barry Roux re-examined he might try and argue that the first shot was a warning shot but the other three were reflex. To try and sort out the horlicks of OP's two defences to culpable homicide. But he didn't)
However, OP has plent of experience of guns, shot them often on ranges and passed an exam on the legal niceties of how to handle them. This is understatement problematic for him.
That all aside, the test for culpable homicide is what a "reasonable person" would have done under the circumstances. I think it will be the failure to shout a clear warning eg "get the fuck out of my house - I've got a gun" that will do for him.
In fact, the failure to shout the warning could be enough to show intent to kill - I.e he didn't shout as he didn't want the person to get out the way etc as he intended to kill them
Either way - the very best scenario I can see is a culpable homicide verdict with a long sentence
I may be being cynical but I can see a shrien dewani "breakdown" in the future. In the hope, at least, of being in a secure mental facility. Not that that will be much better