It is very difficult to predict these cycles as they are based on politics, the mood of the nation, and what economic interests control the Government.
Good President Reagan era broke a twenty year embargo on surplus weapons that started somewhere around 1968. You can look in post WW2 magazines and there were lots of surplus rifles you could buy and that dried up after the Robert Kennedy and Martin Luther King assassinations. The country was anti war, anti gun at the time.
However hateful Democratic administrations since President Reagan have imposed conditions to dry up the supply, you can see that by just going through your magazine collection and seeing what is not being imported anymore, supply has decreased since President Reagan left office.
All we need is a combination of bad Congress, another evil Democratic President and bad media event (Stockton Massacre for the US and Dunblane http://en.wikipedia.org/wiki/Dunblane_school_massacre
for the UK) and private ownership of firearms will end, never mind the importation issue.
I am of the opinion that availability peaked in the Reagan Administration, has been going down since, and in my lifetime, I will never see as many military surplus on the market as I saw in the 80’s.