Have a look at this:
Now have a look at this:
Are these the same?
The first screenshot is taken from this video:
It shows how the NONA self-propelled mortar uses these shells as ammunition.
The (pro-)Russian militants in Slovyansk are using a NONA mortar. You can see it firing here.
The second screenshot is taken from this video:
These are found in residential area's in Slovyansk but did not detonate.
Guess who was shooting these?
(It is unclear why the boy in the video says they are 110 mm, because these are 120 mm.)
Here is a mortar Ukraine is using:
Taken from this video.
This is the same ammunition they are using for their NONA as can be seen in this video:
Conclusion: the (pro)Russians had been shelling residential areas themselves.
Also see: A hospital being hit by a mortar.
Also see: Nona firing from residential area.