This is nothing more than a politically motivated lie.
You cannot escape the fact that guns are overwhelmingly used for peaceful purposes that do not include maiming and killing. If this were not the case, Americans would all be dead or maimed by the guns that outnumber people in our country.
Even where the evidence strictly supports your claim, it counters the intent you imply. FBI standards for selecting ammunition, for example, test penetration through clothing and material designed to simulate a human body, but the intent is to stop lethal threats with a minimum of collateral damage. Quite opposite to being "designed for slaughtering", they are designed to minimize harm, while serving a defensive purpose.
> Via defense or justified actions is irrelevant
I think you'd find the opposite to be the case if a person were threatening your own life.