The horde's armour value is deducted from the number of hits of a single attack. Right?

So, if i'm firing with a gun with ROF 4 in long burst i'll never be able to hit a horde with armour value 5, since the maximum number of hits I can inflict is determined by the ROF.

Am I right?

Where did you get that idea?

Armour Value is just deducted from Damage (though the AV is in turn reduced by the Pen of the weapon used) - it had nothing to do with how many hits you can cause.

To elaborate:

Damaging hordes is a three step process:

1. After hitting, you test if you can wound a member of the horde at all. Make a normal damage roll and if at least 1 point comes through, proceed with step 2. Otherwise you fail to damage.

2. You have hurt the horde, congrats! Calculate the effective number of hits, taking special rules into account.

3. Translate hits into magnitude damage, taking special rules into account and deduct mag damage from horde size.

Done!

Alex