I doubt a shotgun or rifle was used. Those are both long guns, difficult to muffle the sound, not compatible with the silencers allegedly used, unwieldy in tight spaces, and, I apologize for not being able to state this delicately, but in most cases would be expected to cause enough damage that if pressed to the skull the damage would be so extensive that it would be difficult to determine what area on the upper third of the body the next shot targeted, let alone 2 shots.
I also find it unlikely that an automatic weapon was used. Due to legislation, they are more difficult to come by legally without leaving a paper trail. They are harder to control concerning sparing an infant lying between 2 parents who the perpetrators targeted. And, again, the alleged silencers used would be incompatible with many automatic weapons.
A handgun (pistol) would be the most likely choice for tight quarters, multiple targets, wanting to spare all but one child, using a silencer, and maneuvering around each other.
Here is an example of a brass catcher for a pistol.
Catch-All Brass Catcher Pistol
Here is a good read on ammunition, calibers, etc.
https://www.pewpewtactical.com/bullet-sizes-calibers-and-types/#toc0