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The original implosion systems were composed of 12 pentagonal lenses and a layer of the highest impulse explosive available. Nothing could be allowed in the detonation path that would cushion the hammer blow. Later weapons with detonator counts of around 100 did not need or have lenses. The small diameter weapons used in artillery projectiles are implosion systems of two lenses. Gun assembled systems are too wasteful of fissile material (and thus too dirty) for practical use.