This has been done to death here, and on other boards. Anyone who's interested can do some searching. But here are the highlights --
[1] I'm a lawyer, and I know about impugning someone's character in front of a judge and jury, and have participating in doing so (it's part of the job). And I'm also a shooter and NRA certified instructor. I’ve also taken Massad Ayoob’s LFI-1 class.
[2] It's not about the gun or ammunition per se. It's about how certain factors, like tinkering with your gun, using handloaded ammunition, putting "Punisher" grips on your gun, walking around wearing a "Kill Them All and Let God Sort it Our" T-shirt at the mall, or other "gun nut" stuff can be used to attack your character and credibility. This is important because if you're in trial at all, somebody in authority thinks that your claim of self defense is vulnerable, and your testimony might be crucial to establishing that your use of lethal force was justified. If the jury can be convinced by the prosecutor that you're a junior Rambo wannabe, they just might not be inclined to believe your story.
[3] At a trial, at the end of the presentation of evidence, each side gets to argue what the trier of fact should infer from the evidence. So a prosecutor might argue that a trier of fact should infer certain things about your character and disposition for violence from the evidence that you handloaded yourself the ammunition you used. Indeed, I'd expect a prosecutor to conjure up for the jury the image of you up late at night in your garage quietly assembling special super killer bullets.
[4] So Suzy Soccermom now asks herself, as she sits on your jury deciding whether to believe your story about what happened when you shot that nice gang member, why store bought ammunition wasn’t lethal enough to satisfy your perverted blood lust. Remember, Suzi Soccermom and her friends are going to be deciding if the shoot was good.
[5] Yes, we know that there doesn't appear to be a case documenting this, but this would be a trial court matter, and trial court activities are not well publicized or generally published in the official legal reporters. Only decisions of courts of appeals on matters of law are regularly published. In any case, I suspect that the great majority of private citizens who own guns for self defense, including those with CCWs, are not necessarily enthusiasts. They most likely own and carry factory stock guns loaded with ordinary, commercial ammunition. So in fact, it's pretty unlikely that there have been too many cases in which a modified pistol or handloaded ammunition were used.
[6] Of course, if it’s a clean shoot, it won’t matter. BUT why do we think that if we are unfortunate to have to fire our guns in self defense that everyone is necessarily going to agree that it was a clean shoot? Who decides? If we're on trial, someone doesn't think it's a clean shoot, and determining whether or not it was is what the trial is all about.
So you're well trained, did everything correctly and exercised proper judgment. But the physical and forensic evidence is unclear. The witnesses tell conflicting stories. Or for whatever other reasons, the DA or the grand jury has concluded that there is probably cause that a crime has been committed and that you committed it. And so you're held to answer and are now on trial. Now it's not a "clean shoot" until either the judge dismisses the charge or the jury acquits.
Yes there are clean shoots -- instances where the fact that it was legitimate self defense is so clear that things don't go beyond a preliminary investigation. But in real life we can't always count on things working out for us so nicely.
But if you find yourself on trial, someone in authority did not think it was a justified shoot. And this is where the details can get sticky. If you’re pleading self defense, you must, in effect, admit to the act and must demonstrate the factors that justify your use of lethal force, i. e. that you were reasonably in fear of being killed or grievously injured. This can place your perception, state of mind and disposition in issue. You will probably need to tell your story about how you were in fear for your life and had no choice, and whether or not the jury believes you depends in part on the impression you make on them.
[7] Of course, if one is unlucky enough to be on trial, whether or not he used handloads would be only a perhaps small factor. But personally, I'd rather avoid any of these sorts of "wild cards" altogether. Even though I may have an explanation, I know from experience that the less I have to explain, the better off I am.
[8] Whenever someone comes up with a hypothetical to support an argument as to why this isn't an issue, he seems to tell a nice, clean, simple "good shoot" story. Unfortunately, things aren't always that neat. Real life can be messy, and if you're on trial in the first place, the DA or grand jury didn't think it was "good shoot." So try this one on for size.
A guy in a parking lot 7 yards away from you is looking at you and making threatening gestures in your direction with a large butcher knife he's holding in his right hand. He shouts that he wants your money. You're holding your gun drawn in low ready and order him to drop the knife and go away.
At your trial for manslaughter you testify as to that and then say that in response to your order, he begins to raise the knife and move toward you. You further testify that you were in fear of your life, you could not effectively retreat because your back was to a group of closely parked cars, so you believed that you had no choice but to shoot. You shot several times, and he crumbled, dropping the knife as he fell. He never moved again and was dead when the EMTs arrived
However, an eyewitness testifies that the alleged assailant had actually broken off the attack before you fired. He had dropped the knife and started to turn away, and only then did you shoot.
And the forensic pathologist testifies that, based on the entrance wounds and position of the body, he was turning away toward his left when he was shot. Also, given the nature of the wounds, he would have most likely have retained his grip on the knife when he was shot, although it's not impossible that he would have dropped it.
Now your lawyer has to explain or overcome the eyewitness and the forensic pathologist to solidify your claim of self defense. It's also very useful to your case for the jury to believe your account of what happened and the way you perceived things. Do you think that whether members of the jury see you as a solid, responsible citizen or as a gun nut who spends his spare time concocting “special” killer ammo when store bought is good enough for the police would make any difference in how they evaluated your testimony?
Sure, you can explain. But do you really want your lawyer, in addition to having to deal with the testimony of the eyewitness and the pathologist, to also have to try to explain to the jury why handloading ammunition is a good, wholesome hobby? It’s one more thing to explain when there’s already some seriously bad evidence that needs to be dealt with.