The question lies with the wording of the law. If it says "can hold more than x rounds" or "can be modified to hold more than x rounds," you could run into trouble.
It may not matter whether it's meant to hold 10 rounds of .50; if it can hold more than 10 rounds of .223, an enterprising prosecutor could make life difficult.
It's a big problem with laws seeking to place arbitrary limits on magazine size.
Sometimes it’s nice not to destroy the world for a change.