I used a .620 Overkill case and bullet kindly provided to me by fritz454 to look into how a CZ550 and Olympic Arms Ultramag BBK01 action would need to be modified to make this cartridge feed.

Of course the boltface would need to be opened, so we will skip a discussion of that.

Beyond the boltface, the .620 Overkill ALMOST feeds in both the CZ550 and the BBK01.

On the CZ550, the rails would need to be widened just a very small amount; possibly just 1/16 or 1/32" per rail side, and just for a length of about 1 inch. Then the cartridge can be held in the magazine but can still feed up.

Second, on the CZ550 it looks like the front of the feed ramp needs to be lowered by 1/8" or so.

And that's it - looks like a CZ550 would feed a .620 overkill with just those small modifications.

On the BBK601, the rails would have to be widened, and they would need to be opened up a little more than the CZ550 needed.

The feed ramp on the BBK601 would need modification on the top (lowering) and the bottom (radiusing and sloping).

Then the BBK01 should feed fine also.

Both the CZ550 and the BBK601 actions were long enough so no changes are needed there.

Of course, the magazine boxes could be made deeper if desired to fit 3 cartridges down instead of two, but that is an optional change.
I was surprised that the changes to the actions appear to be so minor, and I am very enthusiastic about the project.

500grains,

What is the bolt diamter of the Olympic Arms Ultramag BBK01 action compared to the CZ 550 Safari action? What are some other differences beside the safety? Thanks in advance for the info. Cheers! Ming

The Oly has a .720 bolt, the cz has .700.

The Oly has a Model 70 safety and is pretty true to the mauser design.

Thanks for the info. If this is the case, then the Ultramag BBK01 is a much better action for the 505 Gibbs. It does not matter if the action is a little rough as some polishing work would clean it up nicely. I am more concerned about the metal itself. I was trying to call to talk to the gunsmith at Oly. Arms about the action but have never been able to get hold of him. Later. Ming

500 grains- I totally agree with you on your observations on the CZ550. I made a few .600 Overkill cases and when I tested them in my CZ550 NYATI it also looked like only really minor rail modifications would be necessary. I had to extensively lower my feed ramp when I built the NYATI and at least on mine I don't expect that much more will have to be removed.
I sent the case dimensions to Dave Kiff and we should have the reamer and headspace guages in a few weeks.