There seems to be several bugs related to archers' level 6 class power (reduce energy cost of missile attacks to 800).
First of all it seems to reduce energy cost more than it should and secondly all energy cost reductions from missile weapon skills are ignored. Quick shot and lightning shot reductions are still applied. Also random items can be shot with same energy cost than any missile.

For example level 6 archer without quick shot/lightning shot talents shooting anything (rocks with and without sling, coins, torches) has energy cost of 666.
Level 9 archer with quick shot and lightning shot shooting anything has energy cost of 505.