The S-250 panels are built a bit like a refrigerator. There is a thin outer skin (about 1/32"), then foam filling around some internal "C" channel frame elements, then another layer of 1/32" aluminum. Where the interior skin meets the channel framing, there is a 1/8" thick piece of wood between the inner skin and the channel. The whole thing is bonded by the foam filling.
As I recall, the channels all run vertically. No welding is used - only bonding or rivets.
If you chase down the MIL manuals for the shelters, the manuals show the construction, and how to repair them. The most common damage is someone using a forklift to move them, and harpooning through the side of the shelter.
The shelters make a decent all-weather enclosure - due to their construction, they can be heated with a lightbulb. Air conditioning is a lot easier if a standard room air conditioner is adapted to the shelters in place of the military air conditioner - the MIL model used R22 refrigerant, and has so many components that are piped with flare fittings that they always tend to leak refrigerant. I had one of the MIL air conditioners (from Coleman Surplus) that I bought new in the box, and I could never get it to hold a refrigerant charge for more than a few months. Friends of mine who used the shelters in the military told me that the mechanics were always adding refrigerant, so that was probably normal.
If you buy one, it may or may not have been de-militarized. Sometimes the de-mil was done neatly, but more often, it was done with a sledge hammer or an axe. I have seen some shelters that were surplused in ready-to-use condition, though.
Have fun!
