Lessons/Ideas from D-Day Game
Because we don't want the computations for affecting troops inside bunkers to take too long to determine, suffice to say all small arms firing at bunkers will affect those troops firing back at -1 (regardless of the actual negative points tallied against the troops inside, as long as there are any negatives it will only ever be -1 to their firing), if when firing at the bunker a squad rolls a CnC (star symbol). Otherwise, against bunkers, small arms cannot cause any casualties. Affecting each of the items inside, once the items inside the bunkers fire and take the -1 penalty, all the points are removed and it is set back to zero. Hence the attackers must keep up the fire relentlessly.
Cannon fire hitting bunkers, or bombs, only cause a -1 tally to be applied each each hit if those are the same weight class or greater.
Note also that historically, the Petard Mortar will only affect medium or smaller structures, however, if in automatic range, the door to any bunker is always one weight class lower than the structure itself.
This being said, and as some of the troops themselves commented, if they had had more bazooka's things might have gone differently, that is to say, RPG's.
Refer to the rules for firing into openings as this SHOULD be a reality, but I can find no commentary from soldiers about if any effect occurred from a distance. In the automatic range e.g. 2-3 hexes, all things are different e.g. flamethrowers etc. and all firing can now effect the bunker contents, but outside that range it is less likely or nearly impossible to cause any casualties to occupants or equipment.