Originally Posted by dotKaZE
At level 10, it gives 10%.
Stacked with Rear, it gives +15%, meaning a total of 25%, on top of skills like Headshot's level 5 critical rate (+13.2%), that's almost 40% critical hit rate, maybe more with equipment set up. It's a lot of SP to spend, but I think DFO-World (zzzzz) states that having Critical Physical Hit also gives a 150% damage increase in critical hits (x1.5, not +150%)
Western fire is also adds 120% to headshot's base damage.