A couple of Russian web sources have published images of a .50 caliber (12.7x108mm) anti-materiel rifle said to be domestically manufactured in self-proclaimed Donetsk People’s Republic. They call it an anti-sniper rifle. They also nicknamed it “Donchanka” which means a woman from Donetsk.
According to the mentioned sources, they first started making these rifles using the barrels from NSV-12,7 UTES heavy machine guns. Later they started making the barrels, too. The latter statement sounds controversial. On one hand, there are possibly some manufacturing capacities in Donetsk region which will theoretically allow setting up barrel manufacturing. On the other hand, manufacturing barrels is a pretty complicated process. It is not something that rebels can bolt together from scrap materials. Even if you have industrial machinery, barrel manufacturing still requires proper materials, skills, and a number of special machines.
All other parts of the rifle are also made domestically, which should not be too difficult of a task. Other than barrel the hardest to make part must be the bolt, which I assume can be manufactured with basic machine shop equipment like mill and lathe.
Interesting to note, that anti-materiel rifles are highly desired and demanded firearms in modern war zones. You might have probably seen a lot of images from the Syrian war showing “homemade” anti-material rifles based on the barrels taken from heavy machine guns or autocannons.