Of course, but saying "we saw her driving around the area of the crime, and she bought gun parts months before(and has a good explanation as to why)" is not pro

Of course, but saying "we saw her driving around the area of the crime, and she bought gun parts months before(and has a good explanation as to why)" is not proof - at least in my opinion it doesn't meet the "beyond any reasonable doubt" requirement for conviction by the American justice system. If the experts could establish that the bullet that killed her husband was fired from said gun then sure, that's proof. Asking the jury to basically "guess" is not justice - it's more like rolling a dice.