Bolt Carrier Group. The gas from the departing bullet comes back through the gas tube. From the gas tube to the gas key, down the drilled hole into the space just rearward of the bolt split-rings, which is how the powder residue gets there so fast). The gas causes the bolt to remain pressed hard against the breech, while the BCG starts back. The BCG rearward action moves the cam pin, causing the bolt to rotate and unlock, after which the bolt follows the BCG on it's trip backward, ejecting the brass.
- You've proven the gas is coming from the tube (by using your friend's BCG),
- You know when you pull the BCG back with the charging handle, the BCG moves rearward, the bolt unlocks and follows, so that works.
As suggested above, I'd look for a not-drilled, or clogged hole under the gas key. a very thin piece of wire, bent at a short, sharp 90-degree angle will help you probe around to find or clear a hole, if you're not keen on removing the screws to the key (if you do, remember to stake the screws again, as you'll see).
There may be other reasons for this failure, but the guys above are on the money - it sounds like gas system, and this is the only place/operation that you haven't been able to rule out.
Edit - have you changed any parts...buffer tube, springs, stock, bolt parts? There are lots of parts, all coordinated... anything can throw off the magic.
- You've proven the gas is coming from the tube (by using your friend's BCG),
- You know when you pull the BCG back with the charging handle, the BCG moves rearward, the bolt unlocks and follows, so that works.
As suggested above, I'd look for a not-drilled, or clogged hole under the gas key. a very thin piece of wire, bent at a short, sharp 90-degree angle will help you probe around to find or clear a hole, if you're not keen on removing the screws to the key (if you do, remember to stake the screws again, as you'll see).
There may be other reasons for this failure, but the guys above are on the money - it sounds like gas system, and this is the only place/operation that you haven't been able to rule out.
Edit - have you changed any parts...buffer tube, springs, stock, bolt parts? There are lots of parts, all coordinated... anything can throw off the magic.
