As far as I have seen, the Sig ARs are just a little different than most others. Don't get me wrong, they are great guns and are very well made. But they also have some downsides. My brother has a Sig M400 (the DI Sig) and a Sig 516. At first he just had the M400, but got a 516 because he could not get "other brand uppers" to fit right. After some research he found that if you want to mix n match uppers n lowers, Sigs need Sigs. So he went out and got himself the 516 so he would have an AR he could trade parts with the M400. Now on the other shoe, his Colts, RRA, DPMS, Franklin Arms and Windham ARs (my brother has over 10 AR rifles, its unreal) will all interchange uppers and lowers in a snap. There are also some aftermarket products that don't work with a Sig. Another thing to remember about any piston setup is they use a different Bolt Group than a DI. Sig makes great ARs and handguns. But if you go Sig, you gota mix n match with another Sig. I thought about getting a Sig myself. But it got voted down because if the list of aftermarket that I like on an AR, that wont fit on Sigs (problems with mag-well grips, Batt-levers and a few others). If you really want a piston system AR, buy that Stag or RRA and have a piston added on. Then keep your DI gas tube and BCG on hand as a back-up. I still feel bad about how much you gota pay for ARs over there in NZ. For $3000USD, you could buy 2 Stag or RRA ARs. I hope this helps and Im sure I will feel the wrath from the Sig fanboys for what I wrote.