So far I really like the sand bag/wall idea... If you found a way to segregate the sand into smaller units, potential for sand loss into the domicile would be mitigated pretty well. I won't pretend to know enough about construction to address the moisture issue. Unfortunately, I live in an apartment. I think the best, cheap thing to do is to place your bookshelves in locations where you anticipate fire or where you opposite from where you would be standing. Having read this thread I think I will try to find a good sized trunk or footlocker I can fill with phone books and blankets (maybe even something practical!) to use as a firing position...anything helps when rounds are incoming.
If I had a home, I would certainly want a "safe room", and would try to use the sand bag idea, re-enforced door, secure communications (something that can't be 'cut' or similarly disabled a la cell phone jammer). We have an awesome walk-in closet in this apartment that would work well for the purpose... but it's not one of the 'rooms' that I decorate and it's so full of stuff it's hard to move around in there.
You can have your churches, I'll keep my guns. Just keep your hands off my paycheck and your eyes out of my backyard.
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