Please email or write your senators/representatives. Here is some language I came up with (please feel free to use to urge some congressperson to oppose this):
I just read that policitians from New York and New Jersey introduced legislation in Congress today to ban the online sale of ammunition. I strongly disagree with such a proposed law because (1) it will not stop criminals from obtaining ammunition and committing crimes; (2) it is already illegal for convicted felons to possess ammunition; (3) it will decrease competition in the marketplace for ammunition which will cause higher prices; and (4) this is just another attempt to make it harder for law-abiding citizens to exercise their rights.
While I am in favor of aggresively enforcing current laws against criminals after they commit crimes, we should not pass laws that will just restrict the rights of otherwise law-abiding people. Accordingly, and on behalf of the people of __________, please vote no on this proposed legislation. I will also be contacting Senator _________ to also ask them to vote no. Thank you.