I don't think you'll be able to keep cartridges in the magazine, regardless of what follower you use. You need to start with a .223 action, not a .308. There's more to it than bolt faces and magazine followers.
There are a few items that require modification and/or replacement beside the bolt and barrel, the follower, mag box and follower spring will also need to be changed. All of these can be sourced from Remington. Other than these things, it's a pretty simple job as the action sizes are the same for either cartridge.
Hope this helps.
The rails on a .223 action are approximately 15/16" longer than those on a .308 action. This means that magazine will turn loose of the cartridge sooner in the .308 action, you may lose control of that .204 cartridge before you would like to. Also, the feed ramp is narrower on the .223 action. With the wider feed ramp of the .308 action the .204 round may not be pointed at the chamber, like it should be. Even with the proper magazine box, spring & follower I would anticipate feed issues. But, you might get lucky..... (I double checked my initial thoughts and have these actions, side by side, for comparison. I'm a firm believer in "choose the proper action for the cartridge".)
The feed rails and feed ramp are different. The feed rails and the lips on the magazine box work together, as a system. With the rails being shorter on the .308 action, the magazine will release the cartridge sooner than a .223 action will. The rail isn't there, on the .308 action, to help guide the cartridge toward the ramp which guides the cartridge to the chamber. The larger feed ramp may allow the bullet to hit the end of the barrel instead of going into the chamber. Conversion from .308 to .223, or .308 to WSM, or .223 to .308 or WSM usually isn't worth the trouble. Like I said, you might get lucky! Rifles may be an assembly of "parts", but they're not a 'kit' that can be assembled without serious regard for those parts.
Boy is this news to me, all short action receiver bodies are not the same !
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I wonder if I should get the conversion mag/follower and see if it feeds OK,
or seek a trade with someone, action for action.
It sounds like I could single feed it anyway.
I'm not a Gunsmith but I am more savvy of mechanical stuff than most people. I am in the process of assembling a rifle from all custom parts. It's been enjoyable and I have been getting an education.
Boy is this news to me, all short action receiver bodies are not the same !
***********
I wonder if I should get the conversion mag/follower and see if it feeds OK,
or seek a trade with someone, action for action.
It sounds like I could single feed it anyway.
Those originally made for the SAUMs and WSMs are "different", also. And not just in bolt face diameters. Might be the 'same' on the outside, but different to suit the intended cartridge on the inside.
