Fast-Forward to 2nd range trip:
Just returned home from my 2nd trip to the range with the M&P-10.
Before I went back to the range, I thoroughly cleaned and lubricated the rifle (it wasn't that dirty; I'd only shot 30 or so rounds on the 1st trip). I also purchased a 2nd magazine (also a PMAG; this time, a 10-rounder).
When I got to the range, wanting to eliminate the magazine and any cleanliness/lubrication issues, I started off with the same ammunition I'd used in my 1st trip - Federal XM80C 149g 7.62x51mm. I'd previously identified this as Lake City ball. That was incorrect; this ammo has "FC" stamped on the case instead of the "LC" of Lake City.
I got exactly the same malfunctions as last time. The rifle made it through 6 or 7 rounds, then experienced a FTF jam with the round in the same position as in my photos earlier in the thread. The charging handle was also very hard to pull to the aft position after I removed the magazine.
Having eliminated cleanliness/lubrication as a potential cause, my next step was to try a different magazine. I inserted the 10-rd PMAG and chambered a round. I fired a round, and the rifle jammed, again, with the round in the same orientation. I cleared that jam, chambered the next round, and pulled the trigger. Same result. Now it was clear that this was not magazine-related.
Next, I loaded a full 20-round mag of the Federal PowerShok 180g .308 and chambered a round. The rifle fired all 20 rounds without a hiccup, and locked the bolt open after the last round. This was the first full magazine I've been able to get through with the M&P-10.
Hoping that the rifle just needed some "breaking in", I next loaded 18 or 19 rounds of Aguila 150g 7.62x51mm (I'd fired a couple of rounds of it the other day), chambered a round, and pulled the trigger. The rifle fired but failed to chamber the next round. On removing the magazine and inspecting the next round, it had the same score marks across the top as before, indicating that the bolt had made contact with it, but failed to strip it from the magazine and load it. Tried a few more times with the Aguila, with similar results.
Next, I decided to try the box of Tula steel-cased 150g .308 ammunition I'd bought. I loaded 10 rounds into the 10-round magazine, and the other 10 rounds into the 20-round magazine. The ammo worked flawlessly in the gun from both magazines, and the bolt locked open after I fired the last round from each mag. Incidentally, that Tula must not be flash-suppressed whatsoever, as every shot produced a giant fireball that completely obscured my view through the scope, momentarily!
So, in summary, the rifle worked perfectly with both types (I did only have 40 rounds) of .308 ammunition, and has failed to work with any of the 3 different types (albeit similar) of 7.62x51mm ammo (Federal XM80C 149g, Lake City M80 ball 147g, Aguila 150g).
On the way home from the range, I called Smith & Wesson again and talked to another rep, and asked what kind of ammo they used to test-fire the rifles at the factory. He responded that .308 ammo is used. So, that explains a lot. I asked if the rifle should fire both .308 and 7.62, and he replied that it should, in his opinion. The rifle is chambered in .308 Winchester, which is stamped on the barrel, but, in the owner's manual, 7.62x51mm is listed as an alternative caliber.
So, I've decided to send the rifle back to the factory with a description of the problems, and ask that they do what it takes to get the rifle to run with 7.62x51mm ammo.
Just got an email that they've received the rifle, so, we'll see what happens. I'll post up the results here.