I'm pretty sure bullets are only dangerous due to kinetic energy. So if the Borg could adapt to that energy, the bullets would have no effect against their force fields.
You can't "adapt" to "kinetic energy". There are no voodoo "wavelengths" or "frequencies" you could "remodulate" to. If Borg could adapt to kinetic energy they could also adapt to being stabbed with something sharp or having their skull bashed in. Or to having a rock dropped on them. And that is just retarted.
And even speaking in game mechanis, as Kalavier points out, they don't adapt to mines or being hit with your rifle butt. Because you cannot adapt to that. Because if you apply enough force, their stupid space zombie skulls pop. End of story.
If they did that they wouldn't be able to annoy us to hell and back by forcing us to remodulate like our grandpappy did OVER FOURTY YEARS AGO.
Yes, I think it's dumb.
Grandpappy didn't have a gadget he could point at his weapon, push a button, and be back in the fight in a matter of seconds. He had to run for his damn life to find a safe spot to pull the weapon apart and monkey with a bunch of tools while the Borg ran roughshod over everything he was supposed to protect.
We've seen in First Contact that ye olde bullets work just fine against Borg. In STO Borg also use this awful invention "personal shields" so you have to drop those first, but why Starfleet isn't issuing good old M16s by now is beyond me. Can't adapt to kinetic damage... no remodulation necessary... but nooo, the military geniuses at Starfleet (and also KDF) still prefer to shoot energy weapons at Borg.
One data point is not enough to draw a conclusion. What if the Borg *can* adapt to bullets? We wouldn't know since Picard didn't use a gun enough times.