This is absolute, total, pure and utter nonsense.
A police standard pistol, (Glock 22) has a muzzle velocity of around 1200 feet per second.
An AR-15 has a muzzle velocity of around 3300 feet per second. (almost three times faster)
A bullet traveling from a pistol like a glock 22, will have a tendency to cut a hole through a victim and come out the other side intact, or embed itself in the victim intact - which makes the damage more survivable, because such injuries can be dealt with more easily by medical teams.
A bullet travelling from an AR-15 or rifle, in general is carrying around three times more kinetic energy than that from the pistol, because it's travelling three times faster. The result is that when the bullet impacts the victim, it breaks apart and when that energy dissipates through the target - it causes a tremendous amount of damage, internal organs can be literally blown to bits to the point where they're unidentifiable, essentially - the wounds in question are more closely related to what you'd find on a battlefield, and tend to be far less survivable than from a regular pistol injury.
Don't take my word for it - you can read the account of a Radiologist, who treated the victims of the Parkland massacre for yourself;
https://www.theatlantic.com/politic...land-should-change-the-debate-on-guns/553937/
I put it to you - and anybody who wants to argue it; that an AR15 has no useful purpose in a civilised society, other than the efficient, easy destruction of people.
