This is a reasonable consideration … and anything that Dulles said needs to be taken very seriously. I accept that simply because a decision is prudential does not mean that it has no moral aspect, and there are surely cases where the prudential opinions of bishops need to be listened to very carefully. I don’t, however, think that this discussion about gun control is one of them.
There is no clear line that allows us to identify which issues and which statements on those issues are morally-prudential and which are, for lack of a better term, politically-prudential. I don’t know what the moral question is with regard to gun laws that requires theological analysis and if there is no moral question involved then what is the justification for the bishops’ involvement?
It is surely true that not all political issues contain moral components, so while Dulles’ comment may be true for one issue (in this specific case, capital punishment) it cannot be extended to the degree that it applies to everything. To make a case that it applies to gun control someone needs to identify what is the central moral issue that needs resolving.
Ender