As a user of the Dire Wolf C, I like using its two ATM 6's, as mentioned before, in the ER role.
Let's compare the ATM 6, in the ER role, to a Clan LRM 5 with Artemis, a Clan LBX AC 5, and a Clan Ultra AC 5.
Weapon Weight Ammo/ton Ranges Heat
ATM 6 3.5 10 9/18/27 4
LRM 5 with Art 2 24 7/14/21 2
LBX AC 5 7 20 8/15/24 1
Ultra AC 5 8 20 7/14/21 1
In comparison, IMHO, the ATM 6 is better in the long range role that either type of autocannon because of its much lower weight and better range.
The LRM 5 with Artemis is about half the weight and has twice the ammo and half the heat, but sacrifices range and the ATM 6 can do 1 more point of damage.
For me, the pair ATM 6's on my Dire Wolf C aren't my favourite weapons, but they have range and are a whole lot better than wasting space with AC 5's.