I have a FM 9mm carbine upper that I'd like to use with a mil-spec lower with endomag; but I also want a buffer/spring set-up that will perform with my standard 5.56 upper without changing the set-up (laziness). I've though of some options to keep from breaking the bolt stop with 9mm upper while still maintaining the lower's dual functionality. Would like comments on what would be best option to have good functionality with both 5.56 & 9mm uppers:

1. 9mm standard buffer (around 5 oz.) with .308 carbine spring)
2. same 9mm buffer & spring as #1 with spacer added
3. same 9mm buffer but with standard carbine spring using spacer
4. extended 9mm buffer (around 6 oz.) with standard carbine spring
5. extended 9mm buffer with .308 carbine spring

Thanks for any advice. WB