How are you headspacing,,, it sounds like you are unscrewing the barrel too much. To have the headspace spring fall into the threads of the barrel you would have to unscrew the barrel quite a bit,, it would seem far more that proper headspace would require. Screw the barrel all the way into the extension and back it out slowly till the bolt goes into battery,, two to three clicks more should do it. With the spring all the way into the extension, it should not get anywhere near the threads.
I just modified a 308 spring to 30 06 configurations the other day for a friends build and it works fine.
In thinking this further,, I was headspacing my friends gun yesterday and the bolt just wouldn't drop into battery,, sumpin was up, so I pulled the guts out and discovered that the cocking lever on the bolt was in backwards,, wasn't letting the bolt to go forward and preventing lockup, and also causing barrel to be way out of the extension,, enough to allow the spring to get into the threads,,,