Firearms & Technology
Eberron did not have firearms included when the original material for the 3.5 rules set was published. However, given the flavor of the campaign, the prevalence of research (both mundane and magical) and the effect that the Last War had on most campaigns, this is how I have decided to deal with firearms based on how prevalent they are in relation to the commonly accepted timelines. Feel free to use or ignore as desired.
No Guns (Before 976 YK): For the entire time before the Last War and during the first 80 years of the Last War, there were no guns, nor the very idea of their existence. Up until this point, the presence of magic and its wide spread use provided a much more common solution to situations where significant ranged damage was needed. This would be true until 976 YK.
Very Rare Guns (Default, 976-1002 YK): During the last twenty years of the Last War, Cannith weapon designers were presented with a challenge for significant ranged damage in situations where magic was either suppressed or otherwise too unstable to use. This prompted the very first guns, originally called alchemical cannons, to be designed and tested. However, their application in the Last War was of such limited use, it barely registered a footnote. Sadly, the primary facility researching guns was located in central Cyre and destroyed during the Day of Mourning. None of this information would be common knowledge to those outside of the upper echelons of House Cannith. This would be true from 976 YK up until sometime between 1000 - 1002 YK (after the common start time for most campaigns).
Emerging Guns (1002-1009 YK): In the years after the Day of Mourning, as explorers began to thoroughly examine the wreckage of the nation of Cyre, discoveries concerning the abandoned development of firearms would have been made. Initially, this would have landed in the hands of the Lord of Blades and certain House Cannith recovery specialists, but by 1002 YK, enough independent artificers would have discovered it to allow for it to start becoming common knowledge. However, as guns become more commonplace, the rumors of the Last War restarting become more and more prevalent and realistic. This period would last only a short period of time with an estimated end at 1005 - 1009 YK
Commonplace Guns (1010 YK - Unknown): By 1010 YK, firearms are common enough that every major nation, and many dragonmarked houses, would have firearms and gunslingers immediately available. Whether or not this leads to a restart of the Last War would be dependent on campaign, but it would be certain cause a rise in industrial espionage. This period could last for years or decades based on the nature of the campaign.
Guns are Everywhere (Unknown): Firearms would not be considered to be everywhere until a lasting piece settled over Khorvaire, regardless if it came after another massive confrontation or not. Until that event has occurred, the individual nations and powers in Khorvaire would not allow surpluses large enough to cause this shift to leave their possession.
It is important to note that these conditions are only on Khorvaire. Given the nature of the other continents, the presence of firearms would be at least one step behind Khorvaire and most likely never beyond Emerging Guns. In a similar fashion, the rules for advanced technology in the Technology Guide do not have a direct application in Eberron, as Eberron does not any technologically advanced societies present. However, the rules presented there have been used to redo the implants that Eberron used as grafts. In a similar fashion, advanced magical laboratories are a hallmark of House Cannith, so using technological laboratory mechanic from the Technology Guide can apply, with the exception that effects like Detect Magic and Dispel Magic would apply instead of the technological equivalents. While spells and items that directly apply to technology would not have a function in Eberron, rules, and countermeasures, for radiation can also be applied directly.