First off, thanks for engaging on this. I wish I knew how to post pics, but the image drawn isn't accurate, at least at first glance from how I'm looking at this. I'm guessing the gas port from the key empties into the chamber bounded by the back of ther bolt body at the front end, and the wasp/gas ring section at the rear. When the gas fills this volume, the lugs are still locked, they can't begin rotating until the cam pin engaes the diagonal slot, and since the bolt is locked/fixed at this point, the carrier has to move rearward independently of the bolt.
Now using your picture above, move the red lines, which represent the wasp/ring section of the bolt behind the vent from the carrier. That defines the gas chamber volume, at least the way I'm envisioning it. The gas can expand in all all directions -- gas that expands to the rear pushes against the wasp/rings, but rearward movement is arrested by the locking lugs. Gas that pushes radially against the interior walls of the carrier wouldn't affect rearward carrier movement, I'm thinking anyway. Gas that expands against the rear of the bolt body would tend to initially keep things locked forward I would think, but eventually once the bolt rotates and releases, then the BCG, as a attached unit, starts moving rearward together. But initially, the carrier has to move rearward independently of the bolt (because it's still locked up), so I'm trying to figure out the force vectors that make the carrier move that initial 1/16th - 18th of an inch while the bolt is still locked up.
For the poster who likened this to an automobile engine, I'm not seeing it. Expanding gasses work on an piston that isn't constrained, it simply rotates around a bearing on a crankshaft journal. It's designed to give way and rotate freeley.
An AR Bolt is desgined to remain locked up, big big difference. Inertia, or some other force must kick in and allow the carrier to move rearward that initial bit, but I'm not seeing it. The closest analogy I can see between a carrier and a car's piston is the radial vertical surface area of the carrier, and then I'm trying to figure how the gas gets to that point to act on this area.
ETA: Now looking at the carrier and shining a flashlight down it, I see that within the carrier, there's a shelf where the internal diameter steps down, the part that the firing pin and bolt tail ride through. This may be behind the key gas port into the carrier as you've drawn above, I can't tell for sure, but I'm guessing it is. Then your drawing above is more accurate and please accept my apologies. If this is the case, then I've the location of the gas chamber wrong. It's bounded by the rearward face of the wasp/rings and the vertical face of this stepdown portion within the carrier.
I'd point out two things though: your picture suggests this step down portion presents an angled face to the gas force, an ~30-45 deg face. From eyeballing my carrier, it looks more like a 90 deg angle, but that's just an eyeball WAG. Also, not sure if it makes a big difference. Secondly, the bolt is designed such that the rearmost vertical surface is the wasp/gas ring section, so your drawing should reflect a chamber defined by the end on the bolt at the front (just behind the cam pin hole), an open chamber, and then closed off by the wasp/ring section at the rear. Where the gas vents into the carrier, either behind the wasp/ring section or forward of the wasp/ring into that chamber, I can't tell from eyeballing using my flashlight, but I'm guessing it vents behind. That's where I was getting confused earlier.
Then the rings would be fwd of the gas chamber during lockup, and excess gas wouldn't be vented out the carrier key side holes until the carrier moves independently to the rear. If you shine a flashlight through these holes, you can see that as the carrier moves rearward, it slides past the bolt rings uncovering the holes and any excess gas would be vented at that time.
Assuming the key vents gas into this area, this becomes another vertical surface to act against the carrier and get it pushing reaward independently of the bolt. Once the cam pin hits the angled slot, then bolt unlocking commences. This makes more sense, and if this is the way it works, I now get it.
But now I can't understand why the fwd chamber exists, but I don't want to tax my miniscule brain on that one until I better understand the first question.
2nd ETA: ThomasM4, I should have reviewed your link above. It clearly shows in pictures where the gas vents, and it does vent behind the wasp/rings. So now I get it. Thanks.





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