I found a way to make your cabin essentially bullet resistant on a budget. This should stop most firearms, obviously it wont stop a .50 cal but most commonly used calibers shouldn't penetrate.
3/4 pressure treated plywood
Build the frame of the wall then screw one sheet of 3/4" plywood to both sides of the wall and fill the cavity with gravel. Some people have suggested using sand and gravel but it seems moisture would be a problem in doing this.
Put your top sheets of plywood on then drywall the inside, this is only bullet resistant up to the top of the gravel and is not bulletproof. Bullet resistant means it will take hits, bulletproof means 100%. The studs will be a weak point that ammunition could easily pass. This is only meant to help in making your shelter more resistant to attack it will not make your shelter impervious to damage