This is a DSHK .50" (roughly) caliber heavy machine gun, converted to an infantry support weapon. Typically these guns are mounted to a tripod or vehicle, but Ukraine has been converting these to infantry support weapons by adding a bipod, carry handle, shoulder stock, new handle and new trigger assembly.

For anyone wondering, a DSHK heavy machine gun is 34kg or 75 pounds.

Here’s a video that discusses it in detail, and covers similar american prototype conversions with their equivalent heavy machine, the M2 Browning (38kg, 84lbs).
youtu.be/fd98XcRm0bM?si=MebMTYuxTRKu2cHV