This is stated by Ukrainian investigative blogger Anton Pavlushko on his Facebook page.
"Yesterday there were reports about four detained separatists including a Russian sniper. Today a video of the interrogation was posted online. Two seem to be typical pro-separatist men, one — a local former interior troops serviceman. But the Russian turned out to be an interesting persona. He served as a peacekeeper in the 7th military base in Abkhazia in a reconnaissance battalion, sniper company. Here he served first as a clerk (so smart that he was specially sent from Anapa?) and then in the infantry.
"He started talking about SVD rifle himself: "Well, I did not have an SVD, just a regular automatic gun…"
He learned about the war from 'his guys in the Marine Corps' (former or not is not clear), who went to the Donbas before him.
Video of the interrogation:
IDs of the detainees: