Weapon Upgrade Levels Matter in Online Multiplayer
Aside from your soul level, your weapon upgrade level will also be taken into consideration in online PVP and co-op. If you have a +10 weapon, you’ll only be able to invade or co-op with players who have a +8 weapon or higher, and they must also be within your soul level range. This was likely done to prevent players from creating low level characters with powerful weapons to invade newer players and make their lives a living hell.
From all reports, if your character has a +10 weapon, that character will automatically be sorted into the +10 player bracket even if you end up discarding that weapon. So if you want to invade players at a lower level range, you’ll have to keep your soul and weapon levels low as well – a good way of evening out the online playing field.