Honestly, 700 rounds is probably not enough to foul the gas system to the point that it would fail to extract/eject. Unless it's 700 rounds of the absolutely filthiest, dirtiest, crap ammo that belches smoke like blackpowder.
If you're running blackpowder .223/5.56, then we got a whole other problem. :D
Honestly, I don't think your problem is likely to be in the gas system itself (gas port, block, tube).
So lets start working our way back through the gas system.
This gentleman has a *lot* of knowledge of the AR platform, and he's got a lot of specialized tools that are designed to inspect and gauge the quality of an AR.
https://www.youtube.com/results?search_query=school+of+the+american+rifle+bcgWatch one or two of these videos. You'll see a couple of tests that you can do yourself, at home, without any specialized tools. Those will give you a little bit of an idea on what to look at and check on your BCG.
You many not have the tooling, inclination or desire to check some of theses things the way he does, but take a second and try to wiggle the gas key around and see if it's loose. Test the gas ring tension. Pull the bolt and inspect it, see if there's any loose debris caught in there. Check that the gas path through the gas key is clear (he uses a piece of weedeater string, but anything rigid enough to thread through there will work). Let us know what you find. That would be where I would start.
Look at/clean the upper receiver/bcg. Check for loose debris there like MechAg posted. Check the buffer/buffer spring - see if something has gotten trapped back there that's binding that.
When you use the charging handle to chamber a round, does it feel rough or gritty? does it feel like something is dragging and creating more friction there than usual?
