What constitutes" possesion" of a firearm?

It is mostly what ever the hell law enforcement wants to make it as your NY illustration points out.

possession is in physical control or with the ability to access.

in MOST states property kept in a safe with only one party knowing the combination would NOT be considered possession

However, the burden of proof that the combination is NOT known is going to be on the felon, and how is he going to prove what he doesn't know? So then it comes down to his credibility, and most felons don't have very good credibility.

THAT is why your friend ended up getting busted for possession, the judge/jury didn't believe him when he said he didn't know the combination

I think the acid test for "possession" is, does one have physical control over the firearm. If so, then possession is definitely met.

Edited to add - you are absolutely correct. My explanation only covers what would in my opinion **definitely** constitute possession. There are lots of nuances. Ultimately a court trial may have to decide the issue for someone so charged. If you are a felon and are anywhere near a firearm, then depending on how bad LE wanted you there could be a case made for possession. I too have heard of numerous cases where a firearm was somewhere in a house - not under the direct control - of the felon, yet the felon was still charged with "possession". These types of scenarios unfortunately have to be decided in court.

Want to further cloudy the water look at ***Constructive possession***. Which is what I think you are asking about.

The ATF has established no solid definition, so whatever is deemed as such by the local courts. Classifications will vary from state to state and sometimes even county.