Pros:
Well-polished (especially given the time constraints). Art looks good, music and sfx are welcome additions.
Ingenious system of collecting new weapons. Running back to an enemy's grave to pick up a new gun is dangerous because they are usually surrounded by other enemies. The risk/reward component makes getting a good weapon very rewarding.
The weapon card animations are very nice. And the tradeoff between spam-firing the pistol, so you can use the new weapon versus making every shot count adds diverging playstyles.
Cons:
The knockback on the pistol is a bit strong. I could see this kind of knockback on a much more powerful weapon, but it can get old quickly on the default weapon.
The weapons are fun to use, but I didn't get as much time to use them as I would have liked. A bit more ammo with each would have been nice (particularly the SMG - felt like I only got 1 or 2 seconds of continuous fire out of it, which is a shame, because it is IMMENSELY satisfying to unload it into a horde of enemies).
Hard to tell when I was hit. Could use some audio feedback, some sprite flash, and screen shake when hit. Would greatly improve game feel.
UI for gun cards is a bit small. I'm getting nitpicky
Cool Ideas:
Most arcade shooter ideas would work well here, such as powerups and bosses. New enemy types would go a long way as well. Maybe certain enemies could take away guns from the field or even revive fallen enemies. Others could sacrifice themselves and try to block bullets from special weapons.
Clear indicators for which weapon you get from each grave. This may have been in there already, but I couldn't tell amidst the chaos.
All said, excellent submission. I had fun playing it for the short time I had. That's what counts!