[quote=@Andrew Blade]
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. 
[/quote]

I thought that both the 6.8 and .40 and 10mm came from the... taka taka
https://en.wikipedia.org/wiki/.30_Remington
So you can neck down any larger into a smaller. But not in reverse. That dog wont hunt.

Yes, i was aware that carrier tilt was a general problem plaguing ARs, i thought i'd get away with it for a while and didnt want to make the budget list into indevidual rifle components, thats why i included a littel redundancy, but HEY! I have a CNC that can make carrier shims or whatever.

I notice you hate the .40 :) i dont. And i think ive stolen a bit of a march on you. Heres my thinking. No Civilian or police pistol caliber will penetrate body armour these days. So what do they have to penetrate? A glass windshield for a start. So what do you do when your using a 9mm? Go up to 147 grain so it penetrates and doesnt dowhateveritdoes? But the 9mm loses some of its appealing characteristics when its as heavy as 147, range for one. Do you know what round has all the benefits and none of the drawbacks? Its the [s].357sig![/s] ofco... i mean the .40+P! The .40 already has a projectile thats ~145gr and can fit enough powder to make sure it goes through that glass/wood/cardoor. Infact with a .40 you can make it replicate any caliber from a 9 to .357sig to light 10mm load, without having to switch platforms. I dont hate the .40! I hate the 9!