I'm going to clean the thing and try again tomorrow (but it's only got 100 rds through it so I don't know how it could be sood dirty as to require cleaning to eject a shell, but I'm gonna anyway).
Did you clean it prior to firing it? It isn't unusual for a gun to sit on the sales rack for awhile and to have a thick coating of some type of anti-rust agent in the chamber and barrel in a new rifle. That might explain the sticking. I'd definitely try giving the bore and chamber a good cleaning if you haven't yet.
Definitely try different ammo at well. Remington has the occasional QC issue like anyone else. Try at least one box of Federal XM193 or Winchester Q3131 (or Q3131A). Do you have the product code from the side of the Remington ammo box? Also try a box of Federal American Eagle or some similar .223 FMJ.
What should I look for? I know a little about what to look for, but I don't see anything upon inspection...
Check the reloading forum and they should have some pictures that show you what to look for.
With .223 Remington, you shouldn't be having any overpressure issues though. What type of buffer and stock are you running? If you take the buffer spring out and measure it, how long is it? Try firing one round out of a magazine several times? Does the weapon lock back on an empty magazine.
You have some type of overfunction issue (the chamber is building up more pressure than it should and cycling faster than it can relieve that pressure). Common causes of this are tight headspace, rough chamber, or an improperly cut chamber. However, if there is crud in the chamber that can also cause the problem.
You might check the buffer and buffer spring as well. If they are really weak or the wrong type for the stock, they can sometimes create the same problem because the weapon cycles too fast. You should have a carbine buffer and spring for a telestock. A rifle buffer and spring for a fixed stock.