I am super-duper sorry, Banzai. I love LWRC's stuff, and I was a huge fan of their move into the piston world, but you're gonna have a bad time with an LWRC piston gun after a few thousand rounds. LWRC does not use a BCG/buffer-tube combo that prevents carrier tilt, and after a while, that BCG is going to be scraping against the base of the buffer tube. It might not be a deal breaker for you, but if you're planning on getting them for their durability... In the real world, you would be disappointed after a while.

In addition, the 6.8 SPC does not use .40 casings, not even close. the .40 casing is way way way too short to accomodate the 6.8 round, not to mention the 6.8 is 6.8 mm and the .40 is 10mm in diameter. The 6.8 SPC and SPC II use the .30 Remington case. 

I love the Sig 716 DMR :) They solved the carrier tilt problem by using a proprietary BCG design that floats the bcg over the buffer tube floor.