It's a special riving tool and I've looked it, but have never been able to find it. I've never tried it, but the some use a shop press. You obviously would do that before the flat get's rolled.

 

Thanks for your reply! Yep, I’ve heard of the mystical HK rivet tool, but also have never seen one. If I figure anything out, I’ll post here.

 

It is nothing special, just a rivet buck and hammer. It is easier to do flat, but making a piece to go inside the mag well to support rivet from rear is simple. And I usually weld the case deflector. Makes it more rigid.
To make the rivet head forming tool- take a steel punch and mill a hemispherical hollow in it with an appropriate sized ball end mill. Harden it. A few taps is all it takes.

 

Thanks for the tips!! When I get back into my MP5 builds, I’ll post my results. Thanks again!!