Mass-effect – How to you tell exactly how much an Upgrade augments your weapons in Mass Effect 3

Being below weight capacity proportionally increases power recharge speed. Being above it decreases it. Carrying less guns reduces your weight (thereby increasing recharge speed). It does not affect the amount of ammo individual weapons hold (mods do).

The wiki weapons entry doesn't currently have individual weights for weapons, but likely that information will be added soon. In the meantime on the weapon equip screen as you add/remove weapons it will give you a percentage penalty/bonus to recharge rate. This can give you a base idea, but ultimately if you use a lot of powers you'll want to equip as few weapons as possible and put some points into increased carrying capacity.

Modifying your weapon is a bit of a convoluted process.

First, you need both a modifiable weapon, and a weapon mod. For instance, you can only stick an SMG mod on an SMG - no SMG means you can't use the mod at all.

Second, you need to find a weapon bench, or go to the armory on the Normandy and use that.

From here, simply click the appropriate weapon class, and then the red "Modify" button.

Next, you want to select one of the two mod slots. Note the counter here. If the number is 0/0, that means you have no available weapon mods (at least for this weapon). Pressing the left/right arrow will let you browse through available mods, and the up/down arrow will move between the two mod slots.

Simply manipulate these two arrows until you've got what you want, and then press "Confirm" on the lower left.

Ta da! Brand swanky new gun!