Declining or growing?
One of the real issues I see with the VPC showing decline in the percentage is they did not factor in that the population increased 50%, from around 2 million to 3 million, Now according to this site
In 1973 there were 68,251,000 households gun ownership according to the VPC was 48% so net was 32,760,000 households had guns. In 2014 there were 123,229,000 households VPC says 33% own guns, net 40,667,000. So even if there has been a percentage drop, the overall number has grown, by 25%
Also we have to bear in mind unknown people contacting gun owners by phone or even coming to your door, many are not going to be forthcoming about owning or how many guns they own.