Maybe something like:
A few facts--Makes only base quality weapons, or maybe well-crafted for master or legendary crafters.
General Familiarity--Makes at qualities associated with, say, third to half skill. No exceptionals or masterworks.
Common Knowledge--Makes at qualities associated with, say, two-thirds to three-quarters skill. No masterworks.
Native (weaponry equivilant of Common Domestic in animal training; all weapons your civ has access to by default are at this level, probably)--As what passes for normal.
By simply finding and extensively studying several, say, whips, or chausses, or whatever you're trying to study, you'll eventually get to the "a few facts" level (say, after studying 15-20 of them, destroying 40-75% but those not destroyed can't be re-studied?). At this point, if the weapon is a projectile weapon, you have a chance (50-75%?) to gain the "a few facts" level of comprehension with a random projectile that can be used with that weapon (comprehension of projectiles and weapons is otherwise kept spearate). After creating a couple dozen or so, studying about 50-100 more, or a combination of the two, you get to the "General Familiarity" level of comprehension, at which point merely studying more of them won't help. However, using them will. After a while longer (~80-120 more forgings, or a year and change? of having a 10-dwarf squad use the item), you reach the "Common Knowledge" level. Reaching the "Native" level of comprehension requires a heckofalot of effort, or something other than practice.
Artifacts increase comprehension massively--one launches the comprehension level up past "a few facts" and halfway to "General Familiarity," assuming no former experience with the gear, and an artifact can be studied with no risk of breakage. You can buy gear from traders; this automatically increases your comprehension a bit less than studying the weapon (and then you can do that, too). You can also hire someone from a civ to teach you, which massively increases your comprehension (perhaps to a few facts within a few months, general familiarity within a year, common knowledge in a few more, and native-level by the end of the decade, assuming no other sources of learning?) but also ties up your smiths or craftsdwarves for a while (a total of around 20 days per month with some dwarf with the appropriate skill, to get full benifets), and you need to feed and protect the foreigner.