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Member
I know we have a concurrent SOTAR thread, but this vid is so well done I feel it should be viewed by every AR-15 owner. Shows why things work, why things don't work, and why they break, even in ways we wouldn't have predicted.
It also indirectly explains why "Top tier" is so expensive. Those .001" tolerances aren't cheap.
https://youtu.be/Uv6a1hs9U5o
Senior Member
This was a good video.
Member
Very nice
Quality components, machining and QC isn’t cheap.
It’s what separates Knights from PSA
Senior Member
When thinking about the operation of the BCG, I always thought that as the gas chamber filled it was pushing forward on the bolt itself taking the pressure off the lugs for unlocking...I was never quite sure I understood that that is what was happening. The vid verified for me that that is in fact what is happening.
Senior Member
Handling the BCG and pulling the bolt in and out, watching it turn within the carrier as it does, how could it be any other way? I don't ask rhetorically or condescendingly. I am trying to imagine how the BCG could work without the bolt being thrust forward from the carrier. I can only think of an operating rod pushing back on the carrier, dragging the bolt with it.Originally Posted by TomMcC
Senior Member
0.001"?
The 0.252" bore in the carrier (for the bolt tail) is 0.0003".
On the bolt:
- the tail (.2503 dia) is +/-.0002"
- the bearing ring (.5280" dia) is +/- .0005"
- all seven lugs must have the rear plane within 0.0005"
The bolt is not pushed forward, but the bolt and carrier are pushed away from each other by the expanding gas in the cavity. This may seem like just semantics, but it isn't. Thinking about it in this manner helps understanding how it works.
Since the bolt cannot move, resting against the unmoving rear face of the barrel, when the two move away from each other the carrier must move backwards. After venting the gas from the cavity, the carrier has sufficient momentum to pick-up the bolt through the cam pin and carry it back with it.
Last edited by lysander; 12-08-21 at 20:47.
Senior Member
I don't understand the distinction. If my frame of reference is the carrier then the bolt is pushed forward, is it not? Why is it wrong or inferior to think of it this way?
Member
It’s not wrong to say that the bolt is pushed forward, but that’s not the whole story.
The gas pressure inside the bolt cavity does two things: it pushes the bolt forward, and at the same time it pushes the carrier rearward. When you describe the mechanism as “the bolt being thrust forward from the carrier,” you’re leaving out that second part. For me at least that makes it hard to see why the carrier would ever move. I can appeal to Newton’s Third and conclude that there must be some reaction force on the carrier, but that’s a long way around to get back to the fact that the gas is pushing on the carrier, not just on the bolt.
Manufacturer
It is most accurate to say the bolt and carrier are pushed away from each other.
If the BCG was just floating in space, it wouldn't move much after the bolt extended from the gas pressure.
However, since the bolt is up against a relatively massive object (the barrel and rest of the weapon), the bolt can't move much and therefore the carrier launches backward, propelled by that same gas pressure pushing off against the mass of the whole weapon via the bolt.
Black River Tactical
BRT OPTIMUM Hammer Forged Chrome Lined Barrels - 11.5", 12.5", 14.5", 16"
BRT EZTUNE Preset Gas Tubes - PISTOL, CAR, MID, RIFLE
BRT Bolt Carrier Groups M4A1, M16 CHROME
BRT Covert Comps 5.56, 6X, 7.62
Senior Member
Let's say the bolt isn't moving forward to any degree. Is it still moving enough to relieve the pressure on the lugs making unlocking easier?
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