Absolutely correct. This is a rifle not a shotgun. Look at diameter of barrel. Smaller than victims fingers. Even a .410 shotgun has a larger barrel than this weapon. (A .28 gauge or .20, .16, .12 shotguns etc. are all progressively larger diameter barreled weapons than what's in this picture).
I'm fairly certain that isn't a shotgun, I would say a commercial hunting rifle with a Mauser style bolt action perhaps a Winchester model 70 or something similar.
As to the level of damage a typical hunting rifle bullet will create a relatively clean entry and leave massive internal damage and a ghastly exit wound at normal shooting distances but with the barrel pressed up against your skin or inside your mouth the combustion gases (which may peak at over 50,000 psi) can literally explode your head as seen here.
Typical handgun rounds can generate up to 35,000 psi but lack the volume of gunpowder and combustion gases to get the same effect so a pistol murder/suicide is generally much less messy.