So can modifying ( or repealing ) the 2nd be put on the federal ballot and voted on by all registered voters ???

There is no "federal ballot" generally (all elections are conducted by states), and especially not for Constitutional amendments.

The Constitution can be amended if Congress passes an amendment by a 2/3 vote of both Houses, and such amendment is then passed by 3/4 of state Legislatures. While presumably a state Legislature could ask their citizens by plebiscite or vote whether they should approve the amendment, the Constitution does not require it.

It's really, really hard to pass a Constitutional Amendment.

I'm pretty damn sure that the 2nd would be struck down ( or modified to ban AR's ).

I doubt it. Repealing the 2A isn't often polled, but it almost invariably is rejected by a very large majority. For example:

washingtonpost.com - One in five americans want the second amendment to be repealed national survey finds

Short of repeal, you'd probably never get a majority of people to agree on any specific change to the 2A. Anything narrow will be dismissed by some people as insufficient ("why only AR rifles? Why not other weapons?"); anything broad will run into the same concerns that lead people to reject a total repeal.

Albaby