1) What's with the ammo pack? It doesn't really make sense that it weighs so much but doesn't carry an appreciable amount of extra ammo.

2) How many Navigators are actually from anything other than a good upbringing?

1: Agreed with the Ammo pack, it's weight should be cut in about half.

2: No clue. The Navigators have always sort of been shunted to the side in 40K background fiction. Much like the Knightly Houses.

Well, the Ammo Pack is a bit odd. They should have been more fine-grained about how much ammo for different weapons weighs. On p135 it just says to use 10% of the weapon's weight as a full clip of ammo. Which is okay otherwise, but throw in the dreaded ammo pack and...

That makes a full "clip" (5 shots) for a Lascannon weigh 5.5kg. Or, you could get an ammo pack for 25kg and it'll do you for 80 shots. The equivalent in normal ammo would weigh 88kg! Even if you start an otherwise sensible effort of houseruling, saying heavy weapon ammo weighs some multiplier of basic ammo, you then have other examples such as...

Heavy Bolter: Full clip (60 rounds) weighs 4kg. An ammo pack for it holds 200 rounds, and the equivalent in normal clips only weighs 13.3kg.

Anyone have a more elegant solution than just reducing the weight of ammo packs, or tweaking the amount of ammo they can hold? I like them otherwise - not just cos they'd look cool, especially with a heavy weapon, but just the game effect of cuting back substantially on reloading is nice enough. From experience there were several times when my DH assassin, even with all his skills, was better off dropping one pair of autopistols and pulling out another rather than reloading. I'd hate to think what it's like for a poor lumbering ogryn with his big clumsy hands and gigantic heavy weapon he just loves so much.

As for Navigators: I haven't read much on that section yet. Does the fluff mostly imply they're all from nice stable homes? From what I knew about them before RT came out I got the impression the Navigator Houses were quite well-to-do and respected, or at least put up with since they're in such high demand. I'm sure you could say there are as many Navigators from Renegade Houses as there are from others.

1: I think the ammo pack is ment to be used with weapons like a plasma cannon, were it sort of makes sense and saves a lot of reload time. For an easy to use houserule: Make it work as if it was a simple multiplier both in weight and ammo capacity, plus some set weight. Take the weapons weight divided by ten (or maybe rather divide by 15 or 20? I'm not much good with firearms, but I imagine that would be more appropriate), this is what a normal clip weights. An Ammo Pack will weigh 5 times this, + 5 kgs for the feeding mechanism, the straps, the carry frame etc etc. An ammo pack will also contain 5 times the ammo of the original clip. Be warned though, that I think the ammo pack removes a strategic choice from the combat situations and while choosing weapons.

2: I had a horrible idea: Imagine a nasty RT-family breeding their own bloodline of navigators some where in a dank basement on an offworld. It begun as a sneakily worded contract of service, signed by a scammed navigator 37 generations ago. After a wile the debts had built up so badly that the entire navigator bloodline (or at least a fertile female...) was owned into serfdom. Now they are bred like so much racing horses and their genes are traded on a very very secret black market.

Most Navigators' lives are fraught with danger and sometimes open war. The Navigator houses are constantly fighting with each other for supremacy and this regularly results in violence. It would make sense for a Navigator to be a "Stubjack", though probably not in the exact way it is portrayed in the rulebook.

Since the goal of most Navigator houses is to improve their genestrain, I don't think it would be uncommon for one house to kidnap the promising children of another house - and the treatment of the kidnapped navigator could be anything from lies about his/her origins with kindness to old-fashioned imprisonment.

Just because they have a lot of resources at their disposal (barring a few of the houses) doesn't mean their lives are all feather beds and fine wines.