"constantly redefining machine guns or pistol braces"

Pistol braces was struck down not on second amendment grounds, but because the ATF failed to comply with the Administrative Procedures Act, specifically failing the logical outgrowth test. They proffered a comment period and then did a switch when publishing the final rule.

Similar shenanigans were afoot with the Trump area bump stock ban, which was ruled against by the Supreme Court itself in Garland v. Cargill. I think that had to do with the agency exceeding its authority beyond what the statute specifically specifies. In laymans terms, the legal details were not ambiguous enough to justify the conclusion that the agency came to stretching the statute through their interpretation.