Back to the OPs original request:
Yes, there IS a definition for an "assault rifle" and it has nothing to do with cosmetics.
It's quite simple, really.
An assault rifle is a semi-automatic rifle with a detachable magazine that shoots a medium-powered cartridge and has a maximum effective range of about 300 meters.
A "medium-powered cartridge" is a rifle cartridge more powerful than a handgun cartridge and less powerful than a hunting cartridge.
Thus, the M-1 (.30-06 cal.) and the M-14 (7.62X51) are Main Battle Rifles, not Assault Rifles.
The M-1 Carbine (.30 cal.) original M-16 (5.56 NATO), the AK-47 (7.62X39) and the Ruger Mini-14 (5.56 NATO) can all be classified as assault rifles.
What makes things a bit complicated for most people is that the M-16A2 (with >62-grain bullets), its civilian counterpart the AR-15 and some Ruger Mini-14s have been enhanced and are no longer assault rifles, but high-power rifles.
This occurred when enthusiasts developed the platforms for long-distance shooting.
They had to change to a heavier bullet to reach those ranges, a different twist rate in the barrel to stabilize the heavier bullet, heavier barrels to prevent distortion from heat and a host of other small changes.
The first assault rifle cartridge had a 55-grain bullet.
The second cartridge adopted a 62-grain bullet. A tighter twist rate was adopted at this time to accommodate the even-heavier tracer bullets.
The long-range cartridges use a 72-90 grain bullet in a barrel with an even tighter twist rate.
These long-range cartridges are no longer considered medium-power.
They are also inappropriate to use in stock Ruger Mini-14s made since 1994 because these rifles use a twist rate that doesn't properly stabilize anything heavier than a 62-grain bullet.
Hopefully, this mini-primer will help the OP figure out what all the fuss is about, machinery-wise.
If nothing else, it will give him a start on his own research.
Back to the OPs original request: br br br Yes, ... (