I'd have to measure a rim before and after when using a 600 to believe the rim gets reduced. I'm not saying it doesn't happen, but man I have trouble believing it does. I think more along the lines that the shell isn't dropping down into the collet properly. It could be adjusted too tight or the plunger isn't dropping down quick enough. But then it doesn't do it with other brand shells. Something isn't right. They should all resize just fine.

I was thinking maybe whatever the alloy is they’re using now in those metal base part was softer , but then that wouldn’t make any difference after being sized in the 600 . Anyway I’ll try your suggestions and see what happens ! Thanks :)

I don’t know about the other Mecs but my 16G 600 Jr with the short kit does a poor job of sizing Cheddite hulls. The ring can produce enough friction the rim gets bent. Have the same issue with Cheddite 10 G and a Mec short kit conversion .

No doubt the Mec 600 does fine with American Hulls. Cheddites rims are a bit smaller in diamater new before sizer comes into play. Rim is thinner too. They bend easily when eject /prime force is concentrated on the rim. Not every time mostly when the rim or hull is dirty increasing friction. You can get a rim bend or a dished head from seating the primer hull supported by the rim.

I size decap and prime my short 16 empty’s ( 10 G too) on a PW 375 press, the PW sizer is a FL die like used on a Pistol or Rifle case. Eject is pushed by a custom made rod from inside the Hull that’s releved to clear the fresh primer. Rim never comes into play

Size deprime prime then eject with the custom Delrin ejector rod. Last PW station - eject, pushes on the rod knocking the case out.

Then move the deprimed sized primed empty over to the Mec & finish the job. No worrying about a bad shell in my bag that’s going to need a cleaning rod to eject from the gun.

William