Out of the roughly 300 million guns in the US

[benEzra]

11. 300 million is a lowball estimate now,

and I'd estimate 99%+ are owned by people who can legally own guns under Federal law.

In the few states that require registration of protruding rifle handgrips or restrict post-Civil-War magazine capacities, or in a few jurisdictions where it is easier to get a gun on the black market rather than through legal channels, I'm sure the fraction of noncompliant ownership is higher. I'm not sure I'd lump an accountant with a 15-round Beretta in the same category as an armed robber or violent felon, though, even if you don't see a difference.

I know you want to make that percentage as close to 0% as you can get it, of course...