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One of the problems with producing modern guns or equipment back in the 1860s is that they did not have the precision necessary to do it. Even if you took a blueprint of a modern machine back to the 1860s with you, they did not have the technology to build the machine.
This is a good point, and we forget how things are interconnected. Optics was a growing field, and later on would allow us to take precision photographs of a blueprint, blow them up to scale and cut templates. So for instance, they would do this with the wing design of a WWII aircraft. This process could be a lot faster than doing all kinds of complex measurements.
To the original poster's point, there are a lot of technologies that were complex with low technology that could have certainly continued along their development without other techs taking over. For example, sailing ship technology was advancing rapidly in the 1840s and 1850s. It continued after ironclads, because most of the tech was on civilian ships, but about 1880, it basically stopped, other than increasing scale. But various pulleys and windlasses, etc. were reducing crew sizes, and other technology was making ships more stable or faster platforms that could have improved their capacity in combat, had they not been made obsolete.