The Borg have adapted to Phasers hundreds of times yet over any length of time when the Feds meet the Borg again Phasers have become useful once more. It takes several shots before the Borg adapt to the Phasers again.
When we saw the Borg blasted with bullets and cut up by a sword they aren't something that happened a lot. I mean as far as i'm aware the Borg were shot only the one time by Picard, certainly not long enough for them to adapt. If it takes them a few shots to adapt to phasers which they've adapted to many times in the past then it stands to reason they might require being shot at least a few times before they adapt to projectile weapons.
If Borg drones can create a shield around them then ANY shield regardless of its frequency will stop bullets. For phasers they tune them better but for bullets no tuning is required.
There's no doubt in my mind that the Borg given more time than one single encounter with bullets would adapt and raise a shield.