The machinegun ban portion of that bill was inserted by Charlie Rangel and William Hughes in a questionable voice vote when most of the chamber had gone home for the night. It wasn't a NRA compromise, it was Democrat abuse of procedure.
Back in 1986, I don't think the NRA had anything near the communications network it has now. Even if the NRA had immediately mobilized to stop the FOPA as soon as the machine-gun ban was added to it, what would it have been able to do before the amended bill was passed?
To be sure, I'm not sure the NRA would have been able to kill the Lautenberg Abomination either. On that one, though, it didn't even try.
That seems like accession to a terrible compromise of our most critical rights.
I don't know what the NRA did in the couple weeks of window between the bill being passed and Reagan signing it. Whatever it was, they didn't manage to stop it. I would still ascribe this to a dirty trick by the Dems over the NRA going out with the intention of selling Class III shooters out.