A friend and I are having a rules debate and I would like to get some feed back on this.

Arjac can toss his thunder hammer 6" for a Str10 Ap1 Assault 1 attack. Normally in close combat, a thunder hammer would reduce the target's initiative to 1 assuming they survived the attack. The discussion is about the nature of the ranged attack.

His argument is that the target is reduced to I1 immediately by the ranged attack (since it is a thunder hammer) and thus a Rune Priest could cast Jaws and force the character to have to roll 1 or be removed from play instantly. This would obviously wreck many multiwound high I characters (Greater Deamon of Slaanesh, Abbadon, et al.).

My argument is that there is no mention of the thunder hammer effect in the ranged attack profile. If the designers meant for this to be the case I assume it would read:
"str 10 ap 1 assault 1, thunder hammer"
If my Keeper of Secrets gets hit by it, I believe he still goes at I10 in CC.

Thoughts?