This is a great concept that deserves some more time put into it for sure! The UI is a bit rough so it was hard to figure out what was happening to start with, but once I got into it it was a lot of fun. I think a clearer indication of how your score is affecting what you can buy, or why certain items are available at different times would be good (I had multiple rounds where I couldn't buy a plain block to put a new gun on)
I hope to see more from this concept in future, great work!