"There is no excuse for having a paraphet that is not bullet-proof, since the means of testing it are so readily at hand."
Sir Ernest Swinton, in "The Defense of Duffer's Drift."
Vern, that was the first book I ever read off the Marine Corps' Recommended Reading List. Our platoon traded around an old dog eared copy we had in the company office.
Well, handgun rounds, and each shooting itself, has its own set of variables and at times can defy our expectations. I have been a member of the NTI team for about 1/2 a dozen years now. We meet every month, and run each other through shoothouses our club has permitted us to construct and maintain on its property. I've seen factory .45s deflect off of furring strips, lodge into and not penetrate through 2x4's, and then I've seen small rounds we consider anemic do things to steel plates we wouldn't expect. These are all isolated incidents, but they happen, and when they do, we all take a step back and say to each other, "Hey, look at that!"
Hiding behind bricks would still be my choice over a sheet of plywood. But, reinforcing the door would be of more value, and you can do that to a home already constructed. For most of us, all we need to do is buy a little time to form and execute our defense, not provide so much time that we can sit in a room defenseless for hours while someone attempts to gain access. Replace the strike plate with a longer one, and use screws long enough to go through the door frame into the stud behind it. It dissapates the energy of a strike over a wider piece of the frame behind the door. Do the same with the hinge screws. Get a doorknob that has metal components; most interior knobs have plastic parts. These upgrades will take more than a strong shove to breech the door. If you want to, spend the money for a solid core door; you may want the added protection, or you may decide its not worth the cost. I personally didn't feel the need.
I find the likelihood of an invader committed enough to remain in my home after I've announced I'm armed small, motivated enough to stay engaged and take rounds in a gun battle even less likely. I could envision an intruder chasing me or my wife down before one of us could gain access to an available firearm. So, the exterior and bedroom doors got upgraded with reasonable security enhancements, and anything leading outside stays locked.
I don't need forever, or even 5 minutes. I need only to provide the precious seconds I might not otherwise have.