I must humble disagree with your husband. As a former prosecutor, I have tried at least 2 shotgun murders that I can remember right off the top of my head. Both were very close range shots, 2-5 feet. Both involved birdshot (which is what I have read was used on Matthew) and in neither case was there an exit wound of any kind. Have we heard the gauge of the shotgun used? That will make a little difference, but not much. Really, there is just no way that birdshot will completely penetrate or transect a human body. Just not enough inertia present. And second, birdshot will start spreading out into a pattern immediately after leaving the barrel.
In one of the murders I tried, the victim was shot in the upper back, neck area with a 12 or 16 gauge shotgun loaded with birdshot. He was shot from a range of 2-4 feet. There was a pattern area on his back that covered about the size of a dinner plate. Just dozens of little holes. Birdshot is tiny. Smaller than BB's. There was no exit wound. Even from the ones that went into his neck. The victim died from blood loss. And the ME testified it took about 15-20 minutes for death to occur. And the victim was alive for several minutes before passing out. (same very possibly happened to Matthew).
The second case was a 12 gauge, shot in the chest a 1-2 feet, with birdshot. Again, no exit wounds. In fact he was shot twice. Once he had his arm up in a defensive posture and the shot ripped off a huge chunk of the flesh of his arm, but it didn't penetrate all the way through the arm. He also died of blood loss. You see, birdshot is just too small to cause just massive destruction of tissue like a bullet. Unless you are using buckshot. Or the shot is a contact wound. And then, most of the destruction is caused by the muzzle flash and gas release than the shot itself. Most deaths by shotgun (unless to the head or face) are actually from blood loss. A little pellet nicks an artery or major vessel. And death is rarely instantenous (again unless it is a head shot).
So, it is no surprise that there was no exit wound. Very uncommon in shotgun wounds. In fact exit wounds are not that common at all. Only very high velocity projectiles will have an exit wound if the shot is to center mass. Just too much to penetrate. And it is hard to not hit a bone of some sort.
How sad. Matthew probably knew then, that Mary was the one who shot him.
You are probably right. It may have to do with the gauge and type of ammo used.