The Duck Test
“If it looks like a duck, walks like a duck and quacks like a duck, it most probably is a duck.”
Courts do not elevate form over substance.
The duck test was explicitly called out in the dissenting opinion of Sotomayor, J in Garland v. Cargill 602 US (2024). In that opinion, her honor said that a bump stock was a duck, I mean, a machine gun; of course, the majority looked at the same duck, I mean, bump stock, and decided it didn't really look like machine gun ... or a duck.
This illustrates the limits of the duck test: Different people can look at the same bird and reasonably come to different decisions about whether it's a duck or not.