Read what it says today, not what it said in 1934.
https://www.law.cornell.edu/uscode/text/26/5845
And here's Title 18 chapter 44, the Gun Control Act of 1968
https://www.law.cornell.edu/uscode/text/18/921
You'll notice in the original text any rifle with a barrel less than 18" was an SBR and silencers aren't defined at all.
The current text of the NFA still doesn't define silencers but tells you to refer to the Gun Control Act where they are defined. As written the HPA removes silencers from the NFA but not the GCA.
https://www.law.cornell.edu/uscode/text/26/5845
And here's Title 18 chapter 44, the Gun Control Act of 1968
https://www.law.cornell.edu/uscode/text/18/921
You'll notice in the original text any rifle with a barrel less than 18" was an SBR and silencers aren't defined at all.
The current text of the NFA still doesn't define silencers but tells you to refer to the Gun Control Act where they are defined. As written the HPA removes silencers from the NFA but not the GCA.