I know it is not possible to make comparisons between countries but on that point, the much referenced Australian response to the mass shooting in Tasmania was brought about by a conservative politician tightening gun laws with the backing of the opposition left wing party and the initial UK response to Dunblane was brought in by John Major with the backing (and further legislation) by New Labour.
Perhaps if Sandy Hook had happened under a Republican you might have seen some bipartisan responses to it over there?