What do you suggest me to place into the raider next time I use it? Double piercing mod (same way I used the reegar)?
High Velocity Barrel + Smart Choke for sure. You want that extra range and accuracy. I would use AP ammo for extra double hits on bosses.
I carry a CSMG or Arc Pistol secondary in case I get maps like Hydra.
What's the reason? Ammo crates?
Effective range mostly, but also cooldowns being irrelevant. If an enemies on the opposite site of the map its faster to use cover to quick swap weapons and one shot it with the Arc Pistol than to spend the next 2 seconds running into Raider range.
The Raider is a very high output weapon but against smaller enemies it has a very real range limitation. If there's only 1-2 enemy its just not worth the run.
My Youtube channel with ME3 Vanguard gameplay: http://m.youtube.com/#/channel/UCLcaGVuXEvUt5ne68C1ZlJw