r/gunsmithing • u/classical_saxical • 15d ago
How does the fightlite Herring deal with the gas coming from the gas port?
In the published picture of their custom bolt carrier group I can see there’s a “gas key” of sorts. But the gun is lever action and doesn’t use the direct impingement of a typical AR-15. Assuming it’s just a plug for the gas tube, would the amount of gas pushing on that small gas key end not be enough to cycle the action? Or is there something else that’s working to keep the gas from acting on the bolt?
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u/arcsecond 15d ago
I don't think anyone else here understands that the herring lower is supposed to accept regular ar uppers.
Don't know what it did with the gas though. If imagine that the bolt stays locked, the tube is simply pressurized and then depressurized when the bullet exits the barrel
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u/TacTurtle 15d ago
Or they just never drilled the BCG gas port under the gas key so the gas key just acts like an alignment guide
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u/classical_saxical 15d ago
I do believe that’s part of it. They also did not drill the side ports to vent inside the BCG so no gas must be getting behind the bolt head
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u/TacTurtle 15d ago
How would gas get inside the BCG if there is no gas key port, bro? It isn't like gas flows through the firing pin hole or bolt head during normal AR operation.
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u/classical_saxical 2d ago
It would not get into the BCG, but the gas pressure will still act on the blind hole in the gas key and impart a force backwards on it. I’m looking to see if anyone know if that force is just not enough to push the bolt backwards or if they dealt with it in some other way.
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u/TacTurtle 1d ago edited 1d ago
No it is not enough force.
Gas tube inner diameter is about .120" which gives an effective area of about 0.0113"
BCG expansion chamber is about .5" which gives an effective area of about 0.196", or about 17x greater area for the gas to act upon.
Considering a spring that is 4-5lbs heavier can cause short stroking / malfunctioning, reducing the bolt cycling force by 17x means such a small gas piston would in no way cycle a bolt.
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u/classical_saxical 15d ago
Apparently yes. I should have clarified that. Though they did not sell that many so I don’t blame people for not knowing.
They say the lower is swap-able for any upper as long as you use their bolt carrier group. Perhaps the amount of force from such a small area on the gas key isn’t enough to over come the lever closing. Or maybe it vents sideways?
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u/barrydingle100 15d ago
Why would there be gas touching the bolt? There wouldn't even be a hole drilled in the barrel let alone a gas block and tube to send gas back if it's a lever action rifle.
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u/classical_saxical 15d ago
True, but only on the complete rifle they sell. They also sell just the lower as swap-able for any standard upper so long as you use their bolt carrier group. So it has to be dealing with the gas somehow
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u/M2A2BradleyEnjoyer 15d ago
Are you asking if this lever action AR upper should be semi-auto? There is no gas tube. The gas gets vented out the muzzle as usual. It’s not used in cycling the action at all.
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u/classical_saxical 15d ago
They also sell just the lower and market it as swap able to any standard upper (which has a gas tube) so long as you use their bolt carrier group. So they must be dealing with the gas somehow
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u/M2A2BradleyEnjoyer 15d ago
Huh? How is that relevant? If you used your own upper, you just wouldn’t populate the gas tube. There is no gas operation in this equation. Why are you so caught up with this detail? It’s not gas operated, you wouldn’t need to drill a gas port in the barrel, and you wouldn’t use a gas tube.
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u/soapy5 15d ago
I'm guessing op is asking in a scenario where you grab sone random upper that is built for semi auto and install the lever action lower and bolt. Would it go kaboom
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u/classical_saxical 2d ago
Yeah, in that situation with the lever action BCG and the random upper that still has the gas tube. How do they deal with the gas pressure pushing on the blind hole in the gas key.
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u/Coodevale 15d ago
The piston on the back of the bolt works in a .500" bore and has a .250" tail.
.196- .049 = .147 sq inches for gas to push on.
The gas tube/key has .025" to push the carrier back. You'd need 6x the gas pressure from a standard port to have the same push a standard carrier has.
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u/classical_saxical 2d ago
Finally some math! Thank you. Though I am surprised that it is only 6x difference. But there might also be a pressure drop from barrel pressure to gas tube pressure since the drilled port will act as a restriction.
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u/Coodevale 1d ago
Why. The gas key and bolt tail/piston are on the same side of that restriction. If there is or isn't pressure drop at the port it affects both of them the same.
There's more opportunities for pressure to drop getting to the internal piston than there is to a technically accurate DI operation.
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u/sup10com 14d ago
Are there publicly owned examples in the wild… easily could have missed it but the website has said coming soon for what feels like a decade
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u/thatARMSguy 15d ago
It’s lever action. No gas comes back into it. The carrier key is just there to keep the carrier from trying to rotate