Case-by-case hypothetical situations are not covered in laws. The term "possession" is among the prohibitions for >10rd mags. If you hand someone a 30rd mag for your AR at the range, he/she is in possession of it, are they not?
If we are talking the bullseye range, you're limited to 5 rds per mag anyway. Why take the chance?
You might be conflating possession with ownership.
The law will prohibit ownership of all high-cap magazines purchased when the law is active. Any grandfathered mags should not change legal status if they don't change hands--unless they change hands in a commercial exchange sense (one sells it to another person, which would clearly be illegal). There are gray areas that need to be resolved before this law should even be signed. Let me illustrate:
Possession and ownership are two different things, right? Because while they (a friend or anyone you authorize) may possess that magazine, they certainly do not own it; otherwise, by that logic, they possess (own) that firearm as well. It's the same sense as lending your car to someone: they need to be legally capable of operating that car. With that said, the rules governing a legal (grandfathered) magazine should be commensurate with the rules governing a legal firearm. After all, they are one required the other. People treat them as different things, but that's another discussion.
No case-by-case? That is stupid. It's stupid to think that there are cut-and-dry situations when this is clearly a nuanced topic.
Again, if I am to follow this law, it must be at the very least a law that you can follow easily. People ARE going to say shit like "I didn't know that" and rightfully so, since it's hard to make Po'o to Okole with laws with gray areas. One thing we know for sure that is black and white is this: your okole is going to OCCC even if you didn't know you were breaking a law.