This all depends on how it will be enforced. We were told the same thing in CO, take pics/keep receipts, etc.., but our local sheriff said it wasn't really enforceable and it is not like anyone is asking for your pics/receipts if they see a high capacity magazine. The only way it would be an issue is if they had any other legal reason to find out you had them. Kind of like not wearing your seat belt. You won't be pulled over for it but if you get pulled over for something else they will also get you for not wearing it.from what I read, I believe there is a "grandfathered" type deal with high capacity mags.. with that being said, you have to provide PROOF that the mag was purchased prior to the ban.. I believe a picture dating back from the time the ban is enforced" suffice..
So in other words, make sure to take a bunch of pictures of all your magazines and post it on a forum or Instagram to show proof and date stamp LOL
This is still a pain in the rear but this is all about politics and trying to "look like" you care. If they really cared they would work on the issues that are causing people to commit these heinous crimes. Guns are less accessible today than any other time in US history but people today have less respect for themselves and others than any other time as well. Politicians do not want to get to the heart of the matter because it then exposes what they are trying to ignore.