well adapt to FFH2 and try to imagine proposing wacky handgun / cannon, when some people have fireballs ....Frankly CIV4 has completely skipped the first 3-400 years of gunpowder development and jumped in to the 18th century (muskeeters and safe Cannon) Useable firearms appeared way before them. Anyone interested should look up what a handgun actually was.
you would have to propose a well functionning firearm.
plus : having a unit means you can arm 100 men or 1000 men with it : it should be reliable.
you have to consider that the beaker spent in research is the time experimenting to reach a "safe"/efficient model of firearm. or even just that someone invented handguns.. but the "beakers" is just the king (you) saying : "those things looks fun.. have my scientists look into it to give some to my men"
(before that, you won't know it, but maybe every master sergeant of the champion unit had a pistol... or every champion unit with city-raid have 2-3 bombardes, but it was of a very low impact on the battle so the unit is not considered a firearm welding unit... )
I mean, from the 13th to the 17th-18th, firearms were not available/reliable/efficient for arming whole units, so, very reallistically, civ4 only presents musketman when the technology of firearms allows for arming whole units with it !!!