Whenever I play Swat 4 and join any server (including ones on my own LAN, ping is not the issue) I have horrible rubber banding and issues with things like ammo and mag counts going up and down randomly whenever I fire or reload. Singleplayer and hosting via the game itself works flawlessly for me. I've reinstalled, uncapped fps, tried vsync, none of them have worked in terms of fixing it for me. Any suggestions?

I take it you tried forwarding the ports in the router?

They are forwarded, yes
If they were not it would flat out not work anyway, not cause performance issues