Most weapons of the same level and rarity have pretty identical attack stats (at least in shops anyway), but the actual damage they deal is drastically different. Example: a 100 attack hand cannon deals a lot more damage than a 100 attack auto rifle.
Is there a formula or way to tell how much damage I can actually expect to be dealing with this gun when it has that attack?
Best Answer
So far my data points to this being the right explanation: Impact is a measure of how much to scale the Attack of the weapon by, to determine a per-bullet damage. If you make a plot betwen Impact and bullet damage in weapons with the same Attack (sidenote: all weapons of the same level, rarity and weapon slot have the same Attack) you'll see a correlation. I don't know the exact scaling factors and numbers involved, or whether this changes as you go up towards legendaries/exotics as my data is mostly from the most consistent baseline source of weapons I have (white primary engrams from Cryptarch, which give lvl 15 weapons). I also only own a single legendary and no exotic weapons.
This means that if you're comparing Attack directly you might as well be comparing levels. Impact will give you a rough idea of how large the single-shot damage will be relative to other weapons with the same Attack, but if you're comparing a lower Attack weapon with one with a higher Attack, you'll have to do some quick headmath to determine if the weapon will do the right kind of damage for your usage pattern, based on impact difference and rate of fire. In general at this point it's better to just testfire the weapon.
The data I refer to is available in this google spreadsheet