The markings (if any) on the magazine may come into play as well.
If you have a thirty-round magazine that's marked "5.56 x 45", you own an AR chambered in that caliber, and you *don't* own one chambered in .50 Beowulf, then you're going to have a hard time supporting the claim that it's not a prohibited item.
Yeah, I thought that as well.
But, it all depends on how the law is actually written.
If there is no such requirement to own a weapon of that caliber, I can't see how "not" owning one at the time could be enforceable.
"I'm getting one next week (.50 upper)".
Just raising the issue for discussion, wondering if anyone has actually read the text of the law in jusrisdictions where "high capacity" mags are prohibited.
But more importantly, it goes to the issue of sales for the manufacturers.
What prevents a manufacturer, like Magpul, from continuing to sell ten round magazines " for the .50 Beowulf". None of the magazines I own are marked for caliber.