If Obama decides he can gain a few votes by limiting ammunition purchases he will just issue an executive order and worry about the lawsuits later.
The democrats in congress, along with some RINOs, have been complicit in working with Obama to emasculate congress and allowing him to bypass them whenever he feels the desire.
So now, there are really no more checks and balances. We have three branches of government on paper only. The tyrant does what he wants, and no one in a position of power, responsibility or influence seems to have the guts to challenge him in a meaningful way.