I found there were too many bots in invasion - Tier 1 has been the best for me. There is an optic (XTEN Angel-40) with a rangefinder on it that fits most AR's and SMG's which I have used to scout distances and spots in the normal MP maps - I immediately quit out of some as they are too annoying but in a good game you can get a gun done in a single game if you're lucky. Everyone is doing longshots as well so it kind of helps (if you can beat them to it).
Distances are below if you didnt already have them. I've not got data, but I have found that if I put a longer barrel on there I'm not getting longshots at the distances below - i feel like the longer barrell increases the distance required for a longshot and as such have just been using recoil balancing attachments and larger mags.
- Shotguns: 12.5m
- Pistols: 20m
- SMGs: 30m
- LMGs: 38m
- Marksman Rifles: 38m
- Battle Rifles: 38m
- Sniper Rifles: 50m