Except a double arc from a Raider I with Screed/OE/ACM does on average 5 damage +2 unbraceable, then 4 damage + 2 unbraceable.
Don't get me wrong, I love Raiders, but I think counting on a Raider double-arcing something (at least the way I've been using them, anyways) is probably unwise. It's got a good solid front arc, though, and there's nothing wrong with slapping an ordnance upgrade on there to up the damage if you like (Expanded Launchers being probably the most reliable but definitely the most expensive).
If I'm relying on double-arcing, I'm using Gladiators. Engine Techs set up double-arcs very well and a large quantity of black dice on the sides make the crit-dependent upgrades a lot more reliable.

