Forcefield

The forcefield (tile.ff.name) is only available through setblock, give, and NEI. (command: /give @p hbm:tile.ff).

The tile.ff.name reflects any projectile fired at it, including meteors, buildings, boats, boxcars, and airstrikes. It doesn't save itself if fired on from the inside, because that would just launch the projectile into the block emitting the field, destroying the whole thing in the process. However, the forcefield can't stop mobs or players, making it pretty vulnerable to fire from inside.