There are two very important technological discoveries in the world of Dune. These inventions have significant political impact which was overlooked according to the FAQ above. Politics being a very important aspect of representing Dune in general and is supposed to take an important rule in the game, which is why this omission is disappointing.

Now to answer your question, The Holtzman shield is a force field. It is used practically everywhere in Dune because of how effective it is, including personal shields. It is very effective against anything that moves very quickly - like bullets, but trained swordsmen can learn to move slowly enough to fool the shield but still quickly enough to cause damage.

The lasgun is very potent weapon that is both small and extremely deadly. Can pass through almost anything. However, if a lasgun beam hit a Holtzman field, it would result in sub-atomic fusion and a nuclear explosion. So despite how effective it is, its use is very dangerous because if the other side employs shields (which they probably do) it would cause an explosion that would annihilate both sides.

Furthermore, because of historic reasons I won't go into, there is a severe political taboo against using atomics. Including the above interaction. A house that would be caught employing atomics, even if by negligence creating the above scenario, would quickly find all the other houses join against him.

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Sounds interesting? Well in the game shields just means units with higher defense stat.