I started playing this week and was looking at all the skins in the collection tab but when I go to the store, I only see a small portion of them. How do I see the rest of them?

As the title says, would grinding deathmatch be a plausible way of improving your aim? The games are fast, respawn times are super short, and you are given any gun you want from the beginning. Compared to grinding real unrated games, deathm...