
Forcing people to buy a gun is just as bad as forcing people not to in a free society. 
While I agree forcing folks to buy a gun shouldn't be done, I have to point out that it is a bit better than prohibiting them from purchasing one.
1. "For the common defense" might be one area where a government might make demands of citizens.
2. While you are force to spend your money, you retain the value of your money in the gun you purchased as it remains your property, unlike forced payments to support welfare or Obamacare where you are being forced to give others the benefit of your money.
3. While you are forced to buy the gun, you are not forced to use it for your defense or the defense of others, but you have the option to do so. By being forbidden to purchase a gun, you are also denied the primary means of defending yourself and others.