I'm honestly not sure if there is an "ideal" storage method based upon everything you hear about coin storage. I hear SDBs aren't good for lack of airflow and possible humidity issues. Lots don't have any ventilation. I hear you can't store coins in certain wood cabinets because of chemicals that release in the woods, finishes, etc. The way I look at it is that the majority of these coins are 100-200+ years old and if they have survived less "educated" storage methods in the past they should be fine in my safe that is opened often, has a dehumidifier in the big safe and little moisture packs in smaller.
I agree that it can be hard to find the best storage method. That said, stored in the same enclosed in a air tight space with items covered in chemicals known to damage coins (which guns are covered with)...doesn't seem like the best choice to me.
Yes, of course, but if you use hollow points in both, the 9mm will penetrate further. Likewise, if you use FMJ in both, the 9mm will penetrate further. If you use Glasier ammo in the 9mm and FMJ in the AR, then of course the AR will penetrate further.
Sure there is. A well made vault in your home. Moisture control [check] Fire protection [check] Burglary protection [check] Flood protection [check] Tornado protection [check] Protection from zombies [check] Protection from crooked bank employees [check] The only reason I now keep my bullion in a SDB is because I do not have a vault in my home. Eventually, we would like to have a house custom designed and built for us. If we do, it will have a small vault, perhaps 100 sq ft. It will be in a full depth basement in a corner, with the walls, ceiling and floor being constructed of reinforced concrete. With two walls and the floor having earthen insulation, and the ceiling and two interior walls having at least 6" of cement for insulation, fire would not be a problem. Although installing a vault into a new construction isn't very expensive, there are cheaper alternatives that work for both new and existing construction. A good floor safe installed in a cement floor or a good wall safe installed in a cement wall. Installing them inside cement, especially the floor, offers excellent thermal protection. Moisture can be a problem, but it is not impossible to mitigate.
It is true that most public health experts believe that having a gun in the household causes more risk of harm then the crime prevention risk. http://www.uphs.upenn.edu/ficap/resourcebook/pdf/monograph.pdf Having said that I actually strongly support the right of a person to own firearms. Having coins in the house could change the risk benefit equation. Maybe for the average house it puts you sat more risk but with coins in the house it stands to reason that there would be more home invasions and burglary then average.
I still think the safest way to go is a safe deposit box. But it is a hassle and you cannot really enjoy them daily. (Couldn't enjoy them if they were burned up either, though).
As an aside but related to gun safety: the "Freakonomics" guys did a chapter in their book where they propose that swimming pools are far more dangerous than guns. http://freakonomics.com/books/freakonomics/chapter-excerpts/chapter-5/