Would mag capacity limits, limit the effectiveness of weapons to law abiding citizens? Hell yes. Of course 30 rounds is better than 10. Would it limit the effectiveness of criminals? In most cases no, quite frankly. Besides the magazine manufacturers using existing dies to make their reduced cap magazines, there are millions of Magpul gen 2 30 rounders and Glock 19 17 rounders out there already, which aren't confiscatable in the US and which would exist before and after a defacto ban. They(magpul) are about 7 bucks each now and gun guys buy them by the truckload. The fuckton. A general rule a lot of AR15 guys go by is 30 magazines per gun.
You would probably have as hard of a time clearing the US of 30 round AR magazines as you would clearing it up actual guns period. You have got to understand the level of these 30 round magazines sold per day in the US and the cheap going rate. People buy them by the case.
Would you propose a 30 round cap magazine confiscation from those existing owners, who in some cases have spent thousands of dollars on magazines? These "pre-ban" magazines would remain legal if previous magazine limitations are any indication, and thus still in circulation.