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So this isn't really news, but Brownell's did a cool thing, they made an AR18 upper for an AR15 lower receiver, the BRN-180.
"So what?" one may say, "There's already piston ARs." but after I'll slap you on the mouth, I'll explain that this is better than some insipid H&K or Sig Sauer inbred rifle where a short-stroke piston is crudely grafted onto an AR15 action (because Germans are too autistic to just take the AR15 BCG apart occasionally for cleaning), as this is a proper AR18 action, and by all accounts, a robust and well put together one.
It's got the short-stroke piston, and the rotating bolt, but more importantly it has the square-ish bolt-carrier riding on two steel guide-rods with two springs, and it's made to fit on typical AR15 lower receivers and to function with the typical AR15 fire control groups. Not only does this make for a more robust BCG (the H&K HK416 being prone to wearing out bolts faster than M16s), but it also avoids problems such as carrier tilt and also allows the use of a folding stock without having to reposition the recoil assembly in some retarded manner like the Sig MCX or LR300 rifles, producing excess recoil.
For those who want a more classic AR18 look, they do have that special lower receiver which bears that single forward slant profile, as well as a stock modeled after the original plastic folding stock (neither pictured). There's the short and long uppers, as well as options for .300BLK, hopefully they'll do something cool like .458 Socom or .50 Beowulf some day, just for the fun of it. Hopefully MagPul or some company like that makes a nice plastic fore-end for these guns as well.