Crit chance affects both single and burst mode attacks.
Here crit chance chance is applied for each bullet in burst so you can make partial-crit burst, which will have only some bullets critical but most of them would be normal. If most of bullets are critical then whole burst counts as critical.
Better criticals doesn't give you direct buff to dmg. It gives you bonus to critical roll, and if roll is high you get better dmg multiplier so in a way better dmg. You are also most likely to cause additional effects like knockdown or even knockout (rare but still).