Moogle, I think I see your difficulty. But I wouldn't say that metals and mists are 2 ways of doing the same thing as in your gun analogy. Rather, I would say that when one uses metals, one draws the investiture in, and now has filtered investiture. So if one uses iron, one now has a bunch of investiture inside, but it can only be used to pull on metals. We've seen this before on Roshar, where certain colored gems can be used to Soulcast certain things. That seems to be another example of what I would term filtered investiture.
Mists on the other hand are pure investiture - although it's probably also filtered in that it's Preservation's investiture, but it's less filtered in that it can power all of the Allomantic powers, instead of only one, in the case of metals. So when one draws the mists directly, now there's the unfiltered, or less filtered investiture to be used any way one sees fit.
Now, there are some things about this that I don't fully understand. Why can one be either a 1-metal misting or a full mistborn, but nothing in between? That is, why can one use certain filters to pull in the investiture, but not any others, until suddenly one can use all of them? Why could only Vin draw the mists? Why, when she used a little of the mists, could she only use Allomancy, but when she drew them all she could do whatever she wanted? But the basics of filtered / unfiltered investiture seem to make sense to me.