[Patch 1.0-1.1 ARCHIVE] Mercenary | Commando DPS Compendium

[Gorlough]

Essentially you're good, as long as you spec barrage and terminal velocity (and tracer for sure).
Personally I've made good experience with two kinds of hybrid (arsenal/bodyguard and arsenal/pyro), but the straight 31 build is good as well.
It really comes down to your play style. The 31 and the pyro hybrid are close (if not even) in damage output, whereas the bodyguard hybrid sacrifices some damage to be able to effectively off-heal.

Let me show which specs I'm talking about:

http://www.torhead.com/skill-calc#300rcZMIkbkrdRzfzZc.1
This is a classic 31 build, with increased crit and some bonus to healing and venting. It's a turret build, where you profit a lot from standing still while doing damage.

http://www.torhead.com/skill-calc#300bfR0RZMIkdordRzc.1
The bodyguard/arsenal hybrid. A solid damage build, with off-heal capabilities if you switch your cylinder (usefull if you don't have a healer at hand).

http://www.torhead.com/skill-calc#300ZMI...zcZcGbbz.1
The pyro/arsenal hybrid. Again, solid arsenal damage, but with some more mobility due to the dot missile and the empowered rail shot. Very viable in PvP and if you are kite-tanking.

Again, those are just some examples...
There are some more specs, but essentially - if you're going arsenal - these (adjusted to your own play style) are the specs to go for.