How do cannon shop upgrades work?

[Xenobiologista]

My friend and I recently started and we're very confused as to how Rust's cannon shop works. It's not clear how to select whether you want to upgrade an existing cannon or replace it with a different cannon.

[YetiChow]

When you walk up to the specific cannon, you'll have 2 options: 'e' to upgrade and 'spacebar' to select that cannon (at least those are the default keybinds; not sure about controller buttons.)

Each player can select their choice of cannon and yes both players can use the same one.

Upgrading the cannons improves their damage, fire rate, magazine size.... and I'm not sure but it seems to also improve their crit chance? At least that's what I can confirm from the automatic sentry cannons; I haven't upgraded the others yet. New cannons are unlocked through normal progression, and I'm assuming that each cannon has 10 levels of upgrades given that's what it took to max out the ones I have done so far.

In multiplayer it seems like only the host can perform upgrades and purchases on the island; however the host's upgrades will pass along to the co-op player's cannon too!