If I may provide an opinion I haven't seen voiced in this thread yet:
I believe that the 10,000 j rating is intended to limit high bullet-mass long range rifles like the .50 BMG, and any other round that could serve its purpose, because they have the ability to reach out and touch somebody from beyond the protection of a defensive perimeter and through a protective barrier of insufficient strength.
I believe that the rifles capable of this are being targetted because they would be the single most effective tool for a sufficiently dedictated individual should they decide to prematurely unseat an overreaching elected official, to put it nicely.
If one anticipates a more and more unstable future, the likelihood of such a dedicated person arising amongst the capable (read: armed) population increases.
I believe limiting this potential risk to government officials was the single most important intent behind this particular aspect of the OIC.