[Imp]

I have no idea how it works (dont want to know) but if assault with a DC or suitable weapon assume it increases the chance.
From playing general note on how to assault bunkers.
Move adjacent any attack vs will not have a good chance as moving so do not assualt unless that units goal is to suppress. Depending on the situation if only moved one hex & the bunker is badly suppressed it may be worth risking an assault.
Really for a good chance of success start adj to it hit it with lots of firepower to suppress. Adjacent units can use the W key to select flamethrower or demo charge rather than assaulting. Now if that did not kill it esp if used FT the next attack has a very good chance of destroying it as probably routed or close to.
Doing it like this taking your time means 2 bunker busting type weapons DC FT etc will generally kill it, possibly one. Even if the bunker is very heavy & beyond your weapons capabilities you will start getting kill results on the men inside. It will soon fall but no need to wait just hit it hard & they will run out & surrender rather than wait for the next burst of flame to come through the slit.
The key is static or at the very max 1 hex move firer & suppressed damaged target, much like against a vehicle this improves your chances no end & a grenade can finnish the job.
Getting to eager & going for a kill attack before you are ready means its likely to fail so your unit is suppressed, if he is still suppressed next go thats another turn with a poor kill chance so do not assault it until its badly suppressed.
If you like think of as engineers make there way to bunker then takes them a few moments to get in position near a slit/entrance & send something through rather than a hasty lob.
Guessing on 2 attacks average to kill but not far off & helps presserve DC FT as you dont have loads, hope that helps