Mandrel set - all hand-made of highest quality 3018 grade steal.

Seven piece welded straightening mandrel
Here is a complete seven piece kit from Zayas Manufacturing,  to include all steel pin and welded straightening mandrel for the HK with Heavy Duty truing rails unlike any others ever built. Very solid tool. Making straightening a receiver a breeze. Also included are the welding fixtures to set up the MP5SD shrouds and internal fixtures for the cocking tube supports for the MP5 and MP5SD as well as all your nine series HK's It also includes fixture for cocking tube support to site tower.
Comes in custom made case.
Ideal as a set of tools to be shared by members of a gun club.
Potential source of income for gun clubs
Shipment by US Mail or Courier service - shipment is free if sent by way of US Mail
Price : $2,000
Payment Terms by credit card or money order - call me before you make any payments

Some sample Youtube links on bending Flats.

In this video there is a press, but no mandrels used. You can see the potential for how flats can end up with alignment issues.  In most workshops the use of a helping hammer is often required to help with the alignment and straightening elements of the build. I was in this position myself before I built my first set of "tools for the job".
https://www.youtube.com/watch?v=AI5if5NUklA

And in this link the user has the benefit of a set of mandrels, that are placed within the flats, as the press closed them into place.
You can see that the result is already guaranteed to be straight, aligned and true.
https://www.youtube.com/watch?v=goBEMJwsIo0