Anyone know the probability to proc and the increase damage value?

Does this affect sticky ammo?

Watch the third fleet podcast episode 19. They explain it.

OK, I've been crunching some numbers with this. It should be noted that this skill is really only worth using at -20 affinity. If your affinity is higher than that, just use the +damage rampage skills

Assuming the 25% chance on a neg crit to do 150% base damage:
At -20% affinity this give a 3.94% raw damage increase
At 200+ base raw with -20 aff, it beats out rampage attack 3
At 240+ base raw with -20 aff AND attack boost lvl 4 (armor skills), it beats rampage attack 4

However, it is still probably not the best damage option since I'm pretty sure using WEX and either +aff or +damage will completely beat it out as long as you are attacking a weak point.

If I'm understanding this right, this looks like it only alleviates negative affinity, not turn it into a benefit. If it has a 25% chance to double negative affinity damage, then 75% of the time you still do negative affinity damage.

With 25% affinity damage you either do 0.75x dmg 75% of the time or 1.5x dmg 25% of the time for an average 0.75*0.75+1.5*0.25 = 0.9375x dmg per affinity hit. Critical boost doesn't even change anything, so you don't get something like 0.9x and 1.8x or 0.6x and 1.2x at lvl 3 crit boost.

You're honestly better off just slapping on some affinity to negate the negative than trying to turn this into a build.

The only time you would choose this is if you can't increase affinity but still want the higher base which probably brings it to around 9% better than the selection of lower base 0% affinity weapons. It also becomes useless when someone's Fight palico gives you 30% affinity. Overall, very niche.