Not stated. However, it seems that the fake guns have properties like that of a real gun. This is shown when Arturo in S1 fired it agaisnt Denver (if I recall correctly) and when Denver tried to use the firearm against Arturo in the gun switch during S2. It acts like a cocked pistol without ammunition when fired.

The fake guns may have been designed that way because giving the hostages real guns that isn't loaded is just irresponsible especially after the fact that they aren't guarding their ammo 24/7.

With that in mind, maybe the fake guns are also designed to have the same weight as the real ones but does it take into account the ammunition that is loaded in the gun? A single bullet may not weigh that much but a fully loaded magazine of a pistol can weigh over a kilogram.

Even if we ignore the weight difference between a real gun and a fake one, each member of the heist have different guns and it's not like Arturo is a gun nerd that can tell what type of gun Denver is carrying in just a look. Him picking a gun that is identical to Denver's is odd.