Is this 240V outlet undersized?
Change the 2-pole breaker to a Square D Homeline 30A breaker ($9).   Make sure that cable is at least 10 AWG (8 or 6 will do). 

Then fit a NEMA 6-30 as you intend.   

Then replace those two 15A 1-pole breakers with Square D Homeline 15A breakers.  If they power one cable with a shared neutral, use a 2-pole breaker. 

Those Siemens QP breakers have no business in your panel.  Donate those to someone with a Siemens/Murray panel.  This isn't brand loyalty, the breakers do not fit properly and will arc.