A Whopper is definitely cheaper today then it was 30 years ago.
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I do not which Burger Kings you have been frequenting over the past 30 years.
Free market economy certainly does not apply to the firearm business as there are too many government applied restrictions and regulations.
Also, the volume of gun manufacturing is pretty low when compared to other goods. For example, only about 6 million M1 Garrands where manufactured over a 18-20 year production run. Not many in the world of production.
Over the years, manufacturing cost of firearms have gone down as designs and techniques have improved. Consider why we have Glocks and other "Tupperware" guns today instead of P-08 Lugers and Colt 1903 Hammerless pistols.