Just to interject some facts in case someone thinks the 15+ estimate is accurate - that number is on the "crazy/insane" side of accurate.
The "average" US gun owner has on the order of 2.5 to 3.75 guns each.
These figures can be easily figured by taking the low and high esimates of the number of gun owners (80 million, FBI estimate/ 100 million NRA "guesstimate") vs. the estimated number of guns in the US (250 million to 300 million - FBI and NRA estimates). You can google the methodology of the FBI and NRA to see how they got their numbers and definitions then factor such things as antiques/etc.
A safe and accurate statement would be that the average US gun owner has "about 3 guns."
I would imagine that if the sample were "how many guns does the average THR enthusiast" or "ex marine sniper" own, then it might indeed be about 15. But as an estimate of "average US gun owner," that's a terribly sloppy and poor estimate.