Gawd I wish manufacturers were clearer about their .308 components. I like the Magpul SR25 tube because of the ten position holes. This is the site info: Due to its longer length, this receiver extension is compatible with most DPMS® Gen 1 pattern 7.62x51 firearms with standard length AR15/M4 carbine buffer components as well as aftermarket A5 buffers and springs for use on 5.56x45 AR15/M4 platforms.*

I take this to mean for a .308 I use the shorter AR carbine spring (Sprinco Blue size) and AR15 buffer or can use the same tube for a 5.56 using the A5 spring (Sprinco green) and longer buffer. Is my understanding correct? Seems counter-intuitive to me to accomodate two different size springs in the same tube for different platforms. My brain is cramping.