Foxtrot is right, DPMS chambers are pretty tight when new. Try using some Winchester White box, or something similar. Basic, reliable, and good blasting ammo. Run 100 rds or so, and then clean it really well.

One of my DPMS barrels had a very thin layer of thick, hard grease (think grease packing a gun for storage) in the chamber and needed to be cleaned out. A good chamber brush makes this much simpler. I actually used my .308 chamber reamer to remove the bulk of it, and then scrubbed it out with the chamber brush and hoppes 9. This rifle was worse than you experienced, because it wouldn't fully go into battery, but it could be similar.

Did the brass have any scratches or marks in the outside? Brass is great for showing these issues.

$1100 sounds pretty fair for a NIB DPMS LR308.