Had to put my 2 pennies in
Hey all, I'm new to the forum, saw this thread while looking for load data for this cartridge and it made me smile. I love this cartridge and I am sad that it didn't catch on, although I'm not surprised as it has it's share of faults.
I've owned one for a few years, I built the upper while working at the place that makes them for big green. It's a 20" barrel, deeply fluted, medium contour (think DCM) standard length gas system. Has a mini-miculek comp on it which has been over-bored to reduce back blast. A3 upper as they all have, and a carbon fiber handguard tube which I had to make a one-off barrel nut for.
Now, as to why it flopped:
As many have alluded to, marketing was a big factor. The whole "flies like a .223 hits like a .308" was bogus. It should have read something like this, you and your grandfather meet up at deer camp in the woods, grandpa pulls out his old Savage 99 in .300 savage, and you give a knowing smile then pull out your .30 RAR. Because that's what it is, a .300 savage in modern packaging. But then deer season's over and the camera shows you popping the pins to put a .223 upper on the lower and go target shooting. Of course big green would never pay homage to another company. I'm with whoever says their marketing dept comes from the alzheimer's ward.
Production-wise, it sucks. The larger bolt, which is there because 55,000 psi on the paper thin .450 bushy bolt is nutty, requires more than just a new bolt. The barrel extension is proprietary to the cartridge, as is the barrel nut. Also, the upper receiver is it's own specific upper receiver. Handguard choices are severely limited as the barrel nuts are specific. No a mil-spec barrel nut will not work, nor will any AR-10 nuts. And worst of all, the mags are totally specific, single stack boxes with one goofy winged feed lip and run $45 a piece. These detractions are why the .300 blackout will dominate the non 223 AR world, the .300 blackout has none of these problems, one only needs a barrel and everything else is generic AR-15, including mags.
Reloading sucks because of the proprietary brass, makes it almost like loading for some obscure relic unless you have a good stockpile.
Now all that aside, I love mine. Don't shoot it often but it doesn't disappoint. Rifle holds zero with a 3-9x40 extremely well. I regularly make cold bore X ring shots at 100yds with it. My pre-season check up consists of 1 rd at 25 yds to check rough zero and 1 rd at 100 yds to confirm zero. Then a couple rounds at 200yards to re-acquaint myself with the hold-over points on the scope. Shooting wise, it is absolutely lovely, exceptionally soft recoil and muzzle blast. My wife even enjoys shooting it. You can almost stare the bullets into the target.
Then, when deer season is over, I pop the pins and put my .223 upper back on the lower...
I load 150gr Hornady spire points to 2600-2700fps for deer hunting. Holds 1moa easily. Ran 12 rounds of this with beat up bullet noses, rapid fire on the bench into 1.5" at 100 yds. 125gr Nosler BT around 2800fps shoot same point of impact and are accurate enough for coyote. 110Hdy VMAX's @ 3000fps shoot .5 moa. I tried loading Hornady FTX .30-30 160gr bullets, but I couldn't seat them deeply enough to clear the lands. (funny profile) I want to load some 168gr bthp's, I think they'd make a great 200-300 yard target load for punishment of gong steel. It loves IMR 4198 with the lighter bullets, everything else I use IMR 8208xbr. CCI br small rifle primers.