I do not know what, if any "legal" definition they have come up with for "rate of fire", and IF they do, better watch what it says.
There are two terms commonly used in the shooting world (and this also includes some of the military)
"rate of fire" and "cyclic rate".
And while connected, they ARE different.
Cyclic rate refers to the rate at which the action cycles. usually expressed in rounds per minute (rpm)
Rate of fire refers to the rate at which the weapon can actually BE fired. Time is needed to pull the trigger, change magazines, reload a belt in the gun, and while that is being done, the gun isn't firing.
So, the rate of fire is ALWAYS less than the cyclic rate. (the only possible exception is with a belt fed machine gun, and a belt of enough rounds to keep it firing for a full minute without having to reload. and that has its own problems, such as heat, and feeding issues, etc.)
Keep this in mind when you hear them talk. Your M16 may run full auto at 700-900rpm, but its not humanly possible to fire that many rounds through the gun in a single minute.
Let's take 900rpm (just for the ease of the numbers). 900rnds is 30 (thirty) loaded 30rnd magazines. 900rpm means 15rnds per second, so 2 seconds to empty a 30 round mag. Now add in the human element (reaction time) for the shooter to #1) realize the gun needs to be reloaded, and drop the spent magazine, then #2, grab a loaded one, lock it in the gun, and release the bolt to resume firing.
Even if you have everything prepared, and can swap mags and resume firing, in ONE SECOND, you would have to do it 30 times to get 900 rounds, and that's 30 seconds of your minute used up. So, its not possible to get 900 rounds fired in one minute, using 30rnd magazines.
No matter how fast the cyclic rate is, you can't match that in actual shooting.
(at least not from a magazine fed weapon)
What a bump fire stock does is assist the shooter firing fast, and help them do it longer than most people can do just by rapidly pulling the trigger. And, again, the bump fire while it can be faster than just pulling the trigger, it also cannot match the cyclic rate of the action working.
So, when you hear someone talking about how these "military weapons" are designed to "spit out 900 rounds a minute" (or whatever number they use), remember that while the action may function at this rate (and so "designed to..." is sort of correct) they cannot actually fire that many shots in a minute, so the statement is intentionally misleading. (aka, a lie!)
The great danger to us is that they will use that lie as their justification to create a blanket (and possibly open ended) definition in law, that could be applied to a huge number of ordinary and fully legal firearms, making them illegal or more highly regulated.
You can write a law that says an elephant is a duck. You can put a definition in that law that "proves" an elephant is a duck. With the law in place, you can be prosecuted for not having a duck stamp for your elephant. You can even be convicted of violating the law. But that doesn't make an elephant a duck in the real world, it just means the law is ...stupid.
There is a serious risk here, because the people who will define "how fast you can legally shoot" are the barking legislators. IF they define "legal" by cyclic rate, but then USE the numbers from "rate of fire" to set the limits, then say goodbye to every semi auto there is.
That might just be their plan....