I don't think the USA was all to excessively behind, at the time developments were coming pretty quick and fast. Still Prussia clearly was ahead of them by some years at least with the Dreyse needle gun, an early bolt action rifle design that they had 270.000 of by 1866.That is not exactly a result of tech progression. The American Civil War was using relatively low tech equipment. Europe was most of a weapon generation ahead of the US when it came to guns, most likely because Europe's armies were supplied from the government, while the US's military was expected to be largely raised from conscript infantry with their own guns.
An interesting mention in the article there was how the development of cast steel barrel production techniques is what made mass production of this weapon possible. I'm not entirely sure if that's because steel is indeed essential to make these guns work... or perhaps more likely that the life time of the weapon with to poor quality materials like iron would reduce its usage limit to something near useless.
Still this does make one wonder if many of the more interesting gun technologies of the modern day are basically limited by the availability of at least reasonable quality steel. That would certainly put some demands on being able to produce such steel in large amounts if one wanted such weapons on mass then.