Here is an example of how Congress can try to influence the bureaucracy. In this case, it was not a committee hearing, but simply a letter, signed by 47 Senators, to the Interior Secretary asking him to review the rule against carrying firearms in national parks. The Senators want the national parks to abide by state laws. The rule is under review as a result of the letter.
Should citizens be allowed to carry firearms inside national parks, if they are allowed to do so outside the park?
47 senators want guns in national parks - Yahoo! News