don't they have 7x the population? numbers aren't everything. per capita might be a better rule of thumb.
Iowa's population: 3,062,309
Nebraska's population: 1,842,641
This means that the Iowa gun group members are about 4.8 of every 1000 of the general population, while NFOA members are about 2.7 of every 1000. (This assumes that the 15k and 5k membership figures are correct.)