Magic - Physical/Mana barrier - and spelly questions., How do they work exactly? And other questions. Options

[Mystweaver]

Edit: SR4 not SR*

So as previously noted, not actually played a game for SR4 yet. Still reading rules and trying to forget the SR3 rules that are ingrained in the membrane.

While trying to get my head around SR4 in prep for GM'ing, I've got to the point of tackling spells.

Firstly, I like the way learning works. You pay 5 karma and learn the spell (after you've taken the time to learn it). You can then cast this it ANY force up to twice your magic rating? i.e. gone are the days of learning a spell at a specific force? Think I've read that right. If so, thats a nice simple mechanic. Me likes

Now, onto my actual subject of this post: Physical & Mana Barriers.

So as I read it, if I have a character with say Magic 5 & Spellcasting 5, who wants to put up a barrier, he chooses his force of say 5... rolls 10 dice and gets 3 hits. Therefore the barrier has an armour and structure rating of 3? Or is it 8?

I hope it's 8 otherwise this is rubbish. Could someone please clarify? A barrier of 3 would be broken down in a single shot from a holdout!

I presume I am getting a combat spells mechanics right in my head too...
Same mage casts stunbolt against his target at force 5 again. Again he scores 3 hits. The target has a willpower of 4 and gets 2 hits. The target therefore takes 6 boxes of stun? If the target had managed to get 3 hits, he would have take no damage right?

The drain on this spell is now where I also get a little confused, purely as there is one example in the book that adds a little bit extra that I cant back up. On P184, the example says that you add the net successes to the amount of drain you have to resist... yet under drain and in the example on P204 it does not.

Therefore as it is a stunbolt, the drain is (D/2)-1. Is this 2 or 1 (5/2= 2.5 is this rounded up or down). Do I add net successes this if the target got NO hits on his resistance, my mage would have done 9 boxes of stun to him but would be adding 4 to the drain?

Thanks in advance