There have been others. I join in the disappointment at the sad turn the ACLU has taken. But they have been consistent in their opposition to no-fly lists, as an end-run around due process, and have applied the same argument against proposals to create similar no-gun lists.
They are right in both cases.
Then there was this case, which was mainly 1st amendment, but with strong 2nd amendment ingredients:
The closest I think they've gotten to gun cases is probably this one, which was more of a 4th amendment issue:
https://www.aclu.org/cases/caniglia-v-strom