To follow up on this ^.
(In case you weren't sure).
That means you just get your normal S attack + (normal*) 1d6 v
AV.
A S4
SM with power weapon is rolling 4+1d6 against AV10 (most common rear
AV in the game as ALL vehicles (bar walkers) are hit in the rear armour in cc) and at best, all he can do is glance it (on a 6).
*There are some characters that hit as 'monstrous creatures' with power weapons. These would get the normal
MC (2d6) bonus instead of the normal 1d6.