Can a Rem. Model 788 223 Rem, be opened up to
223 Rem? Can it be done with out pulling the barrel? Will there be any feeding problems?
Thank You!

I have never done a 788 but I have done others. Finish reamer is all that is needed. IIRC there is less than .200 difference in OAL. Will the .223 cartridge fit the magazine? If so I don't know why it couldn't be done. Wouldn't think there would be any need to pull the barrel.

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I've done a few back when the earth cooled for shooting small dinosaurs and I'm just going from memory so don't shoot me if I'm wrong. It should be a straight forward conversion and the mags are the same except for the caliber stamp. The hanger however, may have to be bent """SLIGHTLY""" rearward if the cartridges nose up to the top of the barrel. Because it's straight line ram feed and a small cartridge it feeds like a full diameter bolt and you can get hard smiley stops if the cartridges don't line up just so. You can put paper shims under the front or back of the clip hanger until you figure out what has to be done and then adjust the hanger. Some guys also lightly re-cut the base of the hanger one way or the other in the mill to change the angle. I just THWACKETTA them with a laterally actuated impacting device.

I'm a very violent person.

Chambering through the receiver is not the preferred way but it can be done with an extension. Bear in mind, that you will have a one in 14 twist.
Otherwise, it is a no brainer. Much like our Canadian friend, above.