Actually, i don`t think they`re psykers... and i don`t think their weapons work because they think they work.
Why?
I`ve read a fragment from.. i can`t remember where but the important think was that it had a
techpriest or something like that
thinking about the orks weaponry:
/*Techpriest starts thinking about orkoid weaponry*/
'The machine god makes weapons work'
So why does he make the ork weapons work?
Did he turn on us?
No, it can`t be! so... what gives?!
I`ve got it!
They work because orks have some strange psychic powers that make their weapons and mechs and trucks work!
Yeah, that`s it.
/*Techpriest stops thinking about orkoid weaponry because he found "the answear"*/

This is what happens when theology meets sience.