The (Ohio) legal definition of an automatic firearm ORC 2923.11 (E)
was changed in 2015. The second sentence, about magazine capicity over 30 rounds, was dropped. As of 23Mar2015, it became legal in Ohio to use magazines of greater than 30 round capacity without an automatic weapons tax stamp.
Most (all that I've ever come across) magazines are made where the bottom of the mag can be removed (some easier than others) to allow access to the spring and follower. I'm not a lawyer but the way I read the new C-bus law is the magazine cannot be readily converted to hold 30 or more rounds. To me, this means a plug, which could be easily removed, would not render a larger mag legal. A more permanent modification is required. I read that a C-bus gun store, LEPD, was converting mags by installing a larger follower but, IMO that could be easily replaced with the original which would mean that also wouldn't work. I've recently heard them advertise they will store larger magazines for customers until such time as the new law is overturned. So maybe they have come to the same conclusion I have?
The new C-bus large magazine ban doesn't go into effect until July 1st. So I've got until then to modify or remove my larger mags from the city, and I anticipate the law will be overturned. Hopefully before then.