RE “...Fully automatic machine guns are and were already illegal nearly everywhere...”
No, they’re not. Full auto firearms are not “illegal nearly everywhere” - they’re Class III NFA firearms that have to be registered, and you need a 200.00 dollar tax transfer stamp to own or transfer them. They’re completely legal in 40+ states. Similar situation with suppressors (or “silencers”, which is a misleading term).
RE “...banning something that is already banned...”
They’re not “banned”. They’re heavily regulated, and expensive, but they’re certainly not banned, except in a very few states.
Just for clarification purposes...
Legal full autos that can be transferred to a private citizen must already have been registered by May 1986- no new full autos can be registerered. Since the supply is finite, prices are VERY high. A LEGAL M16 rifle is about $16,000.
So yes, sort of. But Trump wasn’t talking about these.
Pre-1986 manufacture.