[TC Rider]

Another simple possibility is that the shooters are loading either 5.54 or .223 slugs into a 7.62x39 casing as a sabot (just as it would work for a .308 or .30-06). This way several different shooters using AK/SKS-47 rifles would be producing ballistically identical slugs (with no rifling grooves).

I say go for the simpler solution. These AK-74 do turn up as surplus here, along with tons of the russian ammo. I recall seeing plenty of both at recent gun shows.

[meadsjn]

go for the simpler solution

Simpler would have the higher probability. Unless the government decides to start actually telling the public the truth about anything, all we're supposed to do is speculate, and wait our turn.