Theyve said previously that the reason they mostly stay out of the 2nd amendment is that there's already a well funded organization that has made the 2nd their sole focus. The NRA's budget is larger than the ACLUs.. so it makes sense for them to prioritize other constitutional issues.

There's mostly downside for them because a significant fraction of their members are not fans of the 2nd amendment.

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

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:

https://lee-phillips.org/aclu2/

Didn't the NRA help form early gun control laws?

Yes but for all practical purposes it's not the same organization anymore.

The NRA is bankrupt and about to be dissolved.

Its not a pick and choose if they fundamentally disagree with the interpretation of the amendment.