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Many of the guns are modeled after real ones, and the chemical thrower is made of both commercial and personal parts many of them just really heavy tanks. Plus the ammo is extremely heavy. One could speculate this is because Jack is extremely strong as heard on Dr. Suchong's diary. It could also be simply done for gameplay purposes to make it more interesting as in most games which completely ignore weight. Even Call of Duty ignores this principle with ammo and carrying two guns (which is not a common practice due to weight).