It did not take long for Turkey to capture at least one of the crude anti-material rifles that the Kurdistan Workers’ Party (PKK) insurgents were showing off in propaganda photos recently.
The ammunition looks to be steel cased. I don’t know of any steel cased .50 BMG ammunition so I believe the rifle must be chambered in either 12.7×108mm or 14.5×114mm.
[ Many thanks to Tarkan for the tip. ]