Someone may on occasion make it through??? There will be security measures to deal with someone getting through on occasion???
If pilots have guns, then someone will make it through on EVERY flight. That's my point.
The replies about the re-enforced door are a little misinformed in my opinion.
I won't go into details, but if you are familiar with the applicable standards, you would quickly realize that the door can be broken down quite quickly if you put a little thought into it. Furthermore, even though the door is re-enforced the cockpit crew could be injured. (Again, I won't go into details)
There are also a few other ways to quickly get through the door that I am aware of but hopefully, these flaws will be less and less common as the designs are reviewed and fixed.
Another factor is psychological impact. Will pilots be able to stay in the cockpit if they know people in the back are being executed? It doesn't even have to be a gun. How about a knife? Will they just sit in the cockpit or be drawn out? Will the first person they shoot at be a terrorist or a human shield whose body gets pushed towards the pilot as he emerges from the cockpit?