Slam only does 1 point of damage, and misses often. Once you get the bash skill it uses that instead.
Quote:
Barbarians, Ogres, and Trolls can use this racial ability to stun the enemy, which interrupts spell casting.
The Slam ability uses the Bash skill if your class possesses it, and using Slam will raise your Bash skill accordingly.
For classes that do not receive Bash, Slam will only inflict 1 point of damage and will never improve.
Slam shares the same cooldown as Bash and Kick
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Sources:
http://wiki.project1999.com/Skill_Slam
And also, using slam all the time on large races with and without a bash skill.