The general principle is (and should be) that the perpetrator of the crime is responsible for everything that happens as a result of the crime. This would include innocent bystanders getting shot and killed by you defending yourself.
However, as mentioned, this applies to criminal penalties, and civil penalties, are something else.
And, juries can, and sometimes do find fault where none should be found.
As to struggling with the gun, and it goes off....well, if the legal principle is followed, then the criminal is responsible, even if it is your gun. However, a jury might be more focused on who's finger was on the trigger, and why, when it went off. And it would be the criminal's defense teams responsibility to focus them that way, if it was your finger on the trigger.
As already mentioned, the devil is in the details, and it is those details, the specifics of the situation and the applicable law, and the jury's understanding of them that will determine the outcome in court.
All else being equal (and it almost never is) bigger bullets tend to work better.