The current practice to limit capacity on a magazine is that people will often insert a rivet in the back of the magazine that prevents the follower—the small piece that follows the rounds—from progressing lower and taking a higher capacity. Often we'll look for this rivet. You can often see on magazines where the capacity has been expanded that this rivet is simply drilled out. Therefore, it's not a permanent alteration. It's very easily defeated.
The other challenge is the manufacturing of these magazines using 3-D printers, or the import of these magazines from the United States, where they're legal.
I would definitely agree that the prohibition of high-capacity magazines is very important to law enforcement.