Hi @bertuscx
I also did my sniper unlocks on breakaway (the snow map), using the gamemode breakthrough.
When you are on the attacking side and get to the third sector, when the objectives to capture are on the oilrig, you can set up your sniper spot on the hills right of the oilrig and then snipe the enemies that are on the oilrig. For weapon unlocks it should work in solo/coop and also in portal servers.
You could try the experience code ABTZ9F on portal. It's with bots.
For sniper stuff I usually go for Hourglass Breakthrough. Lying at the very back of the spawn as an attacker normally works eventually (I'm a terrible sniper).
It should also work against bots, so it might be worth trying the Portal server listed above by @SharpGoblin.
