I'm a little torn about the notion of choosing one specific weapon before a mission. Obviously there are reasons to want more than one option throughout a level, but I do kinda like the "choose wisely" aspect, even though you don't know what you're getting into beforehand (which is why I'm glad there's no limited lives). Like the difference between "Select Your Turtle" compared to something like Metroid or even Gunstar Heroes.
I'm a sucker for ricochet weapons, but this one felt like it was only useful when you were forced to jump down into contact damage with an enemy, and even then, only like 1 bullet in 20 would hit any enemies on the rebound. I would've liked it more if there was a specific series of angles so that you could make informed shots. Like maybe if the initial shot didn't go straight out, but went up or down (or both!) at 30-45 degrees. The upside would be better "trick shots" from safe (cowardly?) positions, but the downside is that when an enemy is coming straight at you, you HAVE to do an angled shot from a pretty specific distance/location.
Maybe the first couple "artifacts" could've been the weapon mods. Like, get the Big Shot at the end of stage 1, then applying the Big Shot automatically as you enter stage 2, in which you collect the Piercing Shot, etc.. until you have all four. Honestly, it wouldn't even have to be split up into four levels, but it works nicely with the "collect treasure objective, exit level" principle. It could all be one introductory level with four locked doors (I was about to suggest special mod-related switches to open these doors, but that's dangerous territory because it adds another game mechanic and would almost require the ability to change mods mid-level). Arranging the first few sections in linear order, automatically switching to the newest mod before starting the next level, and tailoring those first few levels specifically to showcase each weapon's strengths and weaknesses, basically forcing the player to try each at least once. I used the piercing shot once, screamed about "WHY ISN'T THERE A CROUCH BUTTON" but then remembered that it was an intentional limitation with the weapon, and changed back and moved along.
Money comes in a little slowly. I haven't replayed any levels I've previously beaten, so I dunno if there's really any newly-accessible areas, although I assume there's a few jump-accessible areas. At any rate, money doesn't accumulate fast enough to enjoy the upgrades. I'm sure it's possible to just grind already-beaten levels until you get enough money, but cheaper upgrades with smaller improvements still keep the player engaged with a feeling of accomplishment.
The pop-up orbs that come out of the floor in sk1 are kind of obnoxious. They seem very random and at first I thought it was a bug, because at the time I was also experiencing the "bullet permanently suspended in air" bug. It should either be easier to tell where they're going to pop up, or they should take longer after appearing before they shoot.
I had fun playing it, but once i got to the "sk" levels it turned into kind of a chore. I'd love to see you take on this type of game again, with more levels, a more fleshed-out weapons system (maybe the ability to upgrade each mod individually, even though they're free to switch), and a little more polish in the art.