Possible limits on ATF "letter rulings"
Background: agencies are supposed to follow detailed rules when issuing a "regulation": publish a proposal in the Federal Register, receive public comment, then publish a final rule with preamble answering the major comments. Plus comply with the Paperwork Reduction Act, Regulatory Reform Act, etc..
They also issue informal rulings in specific cases, sometimes by letter. ATF technical issues rulings all the time -- this device is or is not a firearm or machinegun, etc.. In practice since these have internal precedent, they can be cited in future letter rulings. (Or as more than one person has found out, they can be completely changed, perhaps on a personal or arbitrary basis). They aren't opened for public comment, usually aren't cleared by any high official, and don't have to comply with the various reg. statutes.
The White House and OMB have started cracking down on these. Here's Exec Office of the President's guidance. Here's an Executive Order. And here's OMB instructions. (all are pdf) As I read them, they include letter rulings within some requirements of an earlier EO, and require that significant rulings (rather narrowly defined) be posted on the web with opportunity for comment.