Well I've seen a few reports on this acquaintance that has been arrested now but none of them said what this insidious upper part was. But it seems he was not arrested on any charges related to the shooters rifle. It was for violations buying a firearm for himself and lying on the 4473.
Since there is no part of an upper receiver that is regulated in any way and the rest of the items he purchased on behalf of the shooter were also unregulated no charges have been filed for any of that. I'm truly curious what this mysterious "conversion kit" or "component" in another report actually was.
Nothing but the lower receiver itself is a regulated item. Parts for the lower aren't and neither are any other "components" of the rifle. Nor are there any parts that increase the rifles capabilities beyond that of any other semi auto. It could be argued that the drum mag does but my experience with these snail drums has not been positive. They jam with an annoying frequency, increase weight by a lot and make the rifle hard to handle.
About the only thing they are really good for is just saying "hey look at this." Whoop dee doo.
At any rate it seems that the reporting on the rifle he used is just tailored to make it seem like it was some special "military type" rifle with special capabilities. They heard the words "modified" and "component" and just ran with it.
A modification can be anything from putting on a sling, some type of sight(s), there's a long list. But there are no modifications that are available to civilians that truly make the rifle into any more than a semi auto piece.