With the 556xi, can you use the .223/556 BCG with the Russian lower and .223/556 ak mags?  If so would you also be able to put 300blk in .223 ak mags and work well with a 330blk barrel that is.

Thanks

I don't think the Russian lower has been released yet, so I'm not sure anyone knows yet.

Oh, good to know. Thanks for the response. It would be cool if it can.

I believe I heard Sig say you could use AR mags designed for 7.62x39 with the 7.62x39 bolt and barrel and the AR lower.  If that does work then I imagine the other should work, but that's just a theory.

http://www.aimsurplus.com/product.aspx?item=MCPAR762

".223 AK mags" is a bit non-specific, as there are many .223 AK mags with varying levels of compatibility. My understanding is that metal 35rd Galil mags have been used successfully in the setup you're referring to on a Sig 556 upper / 556R lower mashup. I know I've used a 556R upper with a 556 lower and fed a 7.62x39 round from an AR mag. (I have no 7.62x39 AR mags, so I couldn't do a more substantial test.)

Whether the 556xi will keep this (accidental) compatibility, I don't know. It's not an officially-supported mode of operation.