It's not the defense's job to assume the jury will know what the prosecution is saying is horseshit. It's the defense's job to put an expert on the stand to tell the jury that the prosecutors are full of shit. Ammunition, while it shouldn't be, is a huge deal in cases involving guns. The prosecution uses it as a means to convey intent to the layperson, and it always works in their favor because no matter what type of ammunition you use, there's an argument that the defendant chose it with intent. FMJ? You don't care about overpenetration (even though that isn't a factor with this particular rifle and the ammo he was using). Hollow points? You wanted to ensure that the victim could not survive. Steel core? You intended to shoot someone wearing body armor (police). Frangible? You're attempting to obstruct the forensic ballistics investigations.
Either way it's not relevant because Kyle didn't choose the ammo, buy the ammo, or even load the ammo into the rifle. His claims that the magazine was loaded and a round was chambered at the time he had gained possession of the rifle went undisputed, but the prosecution was able to bring up bullet bullshit in front of the jury without the defense doing anything in their power to refute the claims. Common sense or not, that's tainting the jury. The only reason you don't get an expert witness is because you can't afford it, and that was not a problem with this defense. They even said they weren't going to call a use of force expert to testify because "they wouldn't need it."
No jury trial is a slam dunk no matter what the facts are. If you're sitting in a courtroom as a defendant, there's always a chance that you will be found guilty and given maximum punishment regardless of the facts. The attorneys need to do everything within their budgets to benefit their client, and paying people with a ton of accreditations to make that argument for you is expected.