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msorenso,
I found this May 16, 2004 post by Robgunbuilder over on AR.

"The Cz550 action needs to have the ejection port enlarged considerably and the upper portion of the rear bridge opened for clearance on ejection. The ejector must be modified, The rails milled out then fitted by trial and error by hand. The ramp must also be milled very carefully as to not overly weaken the lower locking lug and then reshaped for the .600 OK bullet nose shape. The bolt has to be opened to .645, To do this the bolt face must be annealed, then cut back about .003 and then re-case hardened. The remaining bottom of the bolt rim needs to be cut to allow the case head to slide up under the extractor hook while maintaining just the proper amount of tension. Of couse the extractor has to be properly re-shaped. Finally, the follower needs to be machined to a different profile, the spring changed and guide rails added to the mag box. The box itself needs to be modified(widenend) to more closely match the cartridge diameter and in addition the bottom of the action must be re-machined to match the new mag box shape. While all this is being done, one should change the CZ trigger and a add M70 Like three position safety. I have a set of complete directions available that I will share with those who are actually building a .600OK. I should add that the front of the CZ action ring still needs to be Trued ,however, I do not typically lap the bolt lugs since this is a short range gun and they appear to be case hardened( not interested in cutting through the case)."

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