As all money/scrap-related perks easily ruin the economy, I hang around with over 30k scrap in L.A., but often short on 7.62mm ammo thanks to a minigun. I wondered how useful the Powder Packer perk is on several characters, so I've edited the characters to check it out.
The +10% chance of finding bonus Ammo when looting (party-wide) doesn't normally stack together for a higher chance, but looks like it rather gives you several 10% chances to add extra rounds to found ammo piles. I haven't managed yet to trigger the perk thrice, but least twice so far, what shows my screenshots below:
http://picload.org/image/wcggrdg/wl2-powderpacker.jpg
In that crate, you tend to normally find about 6 rounds of ammo, so it's a pretty boost. I think I'm sold on that perk, as I slowly run out of useful perks after level 35.