Quite an enjoyable little demo, excited to see where this goes next.
A few points I'd like to bring up:
Inventory needs some work, as is, you can only see the first 10 mags for weapons, then you don't know how many more you have (I thought the game capped out at 10, so I didn't bother picking up more after, I only realized I had more than 10 at the end screen)
It's not very clear what the attachments do, I can only infer that the AFG, red dot, and the stock mods improve handling, and the brake improves recoil. You can also pick up attachments if you already have one, I'm not sure if this does anything, or if it's an oversight.
All the other points I have are things I know will be added in the future, such as enemy and weapon variety, and perhaps an expanded inventory for more items other than ammo, like healing or pehaps score items if you want to do some arcade-y stuff.