No, you cannot open the cabin doors with the plane pressurized...
Shooting holes through the skin of the airframe will not cause explosive
decompression, it will just add some more airbleeds to what is designed to
be there, and to the inevitable leaky door seals, loose rivets, etc... If
you shoot enough holes (lots and lots) then cabin pressure will finally sag
off as the escaping air flow exceeds what the engines can pump into the
plane ...

Even if the whacko(s) succeed in completely blowing out some windows and
decompressing the airplane, they are in the same boat with the passengers,
being tied to an oxygen mask and unable to invade the cockpit - that's a
lose-lose scenario for them...

A weapon getting on board will likely come inside the food cart, be stashed
by a janitor, etc., rather than with a boarding passenger... Secondly, a
single gun/knife, or even a couple, will not take over the aircraft now that
passengers know that letting the whacko(s) get into the cockpit means a sure
death...
Cargo planes are a far more likely target for whackos now than heavily
defended passenger planes...

Denny