The magma is like a huge bubble of molten rock. When you boil water, it bubbles because it is turning to steam at the bottom of the pan and that steam rises to the surface because it is less dense than the water it is in. When it starts to boil, you can see bubbles form and start toward the surface but they disapate before they reach the surface because the water cools the steam. When all the water is near boiling temperature, it can't cool the steam bubbles so they start making it to the surface. If you put something in the water, like a wooden spoon, it will provide a path to the surface that doesn't allow full contact with the water so the bubbles make it to the surface easier around the spoon.
Making any kind of contact with the magma, whether drilling or a bomb, would provide the path to the surface. Once the magma starts to flow, the pressure will force more magma to flow and the weight of the crust on top of the magma will act like fingers popping a pimple sending magma high into the sky. Any hole would be like a hole in a dike that quickly gets larger. About like popping a ballon.