Given that the firearm creation occurred some 50 chapters before what I'm writing now, and the result was successfully used for at least one plot-point, it would be a bit impractical to revise it just now. However, I can still keep these notes in case I do an eventual full rewrite; and these comments can be useful for filling in some as-yet-undescribed blank spots, such as helping to explain why none of the other game-pieces have managed to assemble their own modern firearms.

In case it was buried where some LessWrongers here didn't read it, the point in the story that goes into the most detail about arranging for the creation of a firearm is in chapter 13, and reads:

"There is a certain project I had been meaning to get started, but which I haven't been able to figure out how to introduce under the auspices of either The Dairy's public or private sides. But as a quiet royal initiative..."

She nodded in understanding, and I went on. "It involves the production of a certain dangerous alchemical substance. Improper care and handling can easily lead to the loss of a hoof, or worse - but when finished, it has a number of applications. The final product consists of about two-thirds cellulose nitrate, which is produced by exposing cotton fibers to an equal blend of sulfuric and nitric acids; about one-third glyceryl trinitrate, which is made by adding glycerol to a similar acid mix, subsequently gelatinized with ether; and about one-twentieth part paraffin or petroleum jelly. There are a number of precautions which need to be taken in order to prevent accidents of various sorts..." I went on describing how to manufacture a generic sort of cordite from raw ingredients - sure, black powder was a lot easier to throw together, but why settle for an inferior product - and wondered whether it would be a good idea to also have Luna be in charge of putting together a hoof-compatible pistol and individual cartridges... but decided to handle that end of things myself. Most likely, I could get away with having a few lab-techs in the Dairy get the individual pieces put together, compartmentalized so none of them knew what the whole project was. I might have to get a bit of help figuring out a decent trigger which could be aimed and worked with hooves, but had a few basic ideas based on the chest-mounted cameras I'd seen in use.

So, trying to take as a given that the single firearm was, in fact, successfully built, then the next time I write about it, I could try describing the additional difficulties currently being described, and how overcoming those difficulties was accomplished.

For example, at that point in the story, the main character had access to the capital city's top engineers and technicians, as well as the local magically-enhanced forges and manufacturing techniques. So while the gun itself may be entirely non-magical in its operation, Equestria might not be able to manufacture it without applying magic during the metal-casting processes. (Or something of the sort.) In general, Equestrian technology is roughly in the 1850's-1870's range, with various exceptions (eg, sound systems, airships) that often turn out to have a magical base.

Another background detail that is potentially useful for such not-quite-retconning is that the Princess to whom all this was being described has been shown to be able to collect information by appearing in other people's dreams. It's possible that she used this technique to help gather details that the protagonist didn't know she knew, to help fill out any blanks required for proper manufacturing.