I have won some military contracts for electronics that had "no dissimilar metals" requirements. We had to go through salt fog, and we had lots of dissimilar metals, but we sealed them.
I have had dissimilar metals problems in plumbing.
But in rifles, I have made pillars from brass [higher than the steel receiver in the Galvanic series] and pillars from Aluminium [lower than the steel receiver in the Galvanic series], and I have never had a problem.
But the rifles I have been building just see sage brush and rain.
My brother bought a working Rem 11 shotgun for $50 that had been used for decades to hunt ducks on the salt water of Hood's canal. When we took it apart, there was lots of rust.