Borderlands – How does Gunzerker perform a melee attack? Which controls for xbox 360

You can get em infinitely but the amount you have to spend is totally random. It could be 100 or 1000000 but you should leave and enter the town after you get one so you don't get the bug seen in Schottlicious' answer.

Damage is first calculated from shields, then the remaining damage is calculated by health.

For example you are using shock sniper, your damage with the sniper is 1000 (including all the bonuses and modifiers) but the damage is further multiplied by it's effectiveness on shields x2 on normal and x2.5 on true vault hunter.

Assuming the target has 1000 health and 1000 shields, you only damage the shields, but not the health, since the damage multiplier applies only to the shield damage and not the health damage.

If the target would instead have only 500 shields, the shields would take 2000 or 2500 damage and the remaining 500 points of damage is subtracted from the health (Only 500 points of damage from 1000 is multiplied with the shock bonus, since the target has only 500 units of shields)

This same rule applies to all elemental damage.

The elemental damage coefficient multiplies the adjusted weapon damage following the formula below:

Final Damage = (Base + Skills + BA Rank + Class Mod + Relic) * (Gun Base) * (Elemental multiplier)