We've had a lot of fun with the recent study claiming that there were 90 guns for every 100 Americans. Now let's point out the truth: That number has got to be waayyyy low. Think about it: How can they make any reasonably accurate estimate? Guns are pretty durable artifacts; they don't just dry up and blow away after X number of years. (I have 120-year-old relics that are still in excellent shape and quite shootable.) Hardly any record-keeping at all was required for firearms in this country prior to 1968, so anyone claiming to know a total number of weapons extant by extrapolating from current sales or production figures is talking through her hat at best or, to use the scientific term, just making stuff up.
As most readers know, one of my hobbies is collecting old military rifles. Maybe a quarter to a third of my rifles show the importer markings required by the Gun Control Act of 1968. This means the remainder found their way into the country by a variety of means before that date. Some may have been imported by big surplus houses like Bannerman's, some may have come in via the dufflebags of returning GIs, and some may have come in individually thanks to tourists. Any attempt to make a meaningful guess as to how many guns like this are in the country is just that; a guess.
My other love, old S&W revolvers, would be just as hard to count using current sales numbers as any kind of marker. The mean date of manufacture for revolvers in my collection is 1968, the year the current system of Federal Firearms Licenses and their attendant sales restrictions came into practice. Prior to that date, there was no requirement to keep retail-level records of guns sales. Some of these guns have never had the slightest attention paid to them by the government at all, having gone from the manufacturer into the mail, from the mail into the owner's sock drawer, and from the owner's estate into a private collection. Once again, any claim to know how many of these old guns linger out there is idle speculation that can't be dressed up as science no matter how starched the lab coat in which we outfit it.
No doubt this study made someone feel better, but its statistical underpinnings make Birnam Wood look deeply rooted. This is why gun confiscation schemes are always so funny. Not only do we not know where they all are, we have no idea how many to look for. I hate to break it to the more idealistic folks that think some kind of magic complete gun ban would somehow solve anything, but this genie is well and truly out of the bottle. You might as well try rounding up sand.