Why the third hole in AR bolt carrier?

[Uni-Vibe]

On the side of the bolt carier you have the two gas vent holes. But aft of that there's a third hole that opens into the area where the firing pin resides. What's that third hole for?

[John Hwang]

On the LMT Enhanced bolt carrier groups, there are 3 holes in the carrier. It's designed to divert more gas out to reduce felt recoil. I haven't noticed any benefits with the 3rd hole from my personal experience. Maybe others can chime in.

[Paulo_Santos]

I just thought that was where the excess gas was vented out from.

[Bad Hammer]

That third hole is for lubing the tail of the bolt......a drop of lube in each vent hole, and a drop in the "third" hole.
At least that's the way I've always done it.

[M. Gale]

I just thought that was where the excess gas was vented out from.

Excess gas vent!

[Ryo]

Excess gas vent. Let's 3 hole carrier is not recommended for AR's with short barrels, ie 10.5

[Bad Hammer]

OK, somebody help me out here.
The third hole that Uni-Vibe is talking about is back where the tail of the bolt resides.
The bolt tail fits in that recess pretty darn tightly. This area is a bearing surface after all.
The hole in question isn't ported into the expansion chamber of the carrier where the gas does it's job.
Since high pressure gas will seek the path of least resistance, things would have to go very badly very fast for any
"excess gas" to be forced through the bolt tail to carrier tolerances and vented out this particular hole.
So I am having difficulty seeing where this hole is, by design, intended to port excess gases.
Again, if I'm wrong, please educate me, because I'd like to correct my own thinking if I'm looking at it incorrectly.
Life is, after all, a learning experience.

[M. Gale]

There is actually a counter bore in the journal of the bolt carrier that supports the tail of the bolt. The "minor" diameter (section closest to the bolt rings) acts with the bolt tail to seal the back of the terminus. The gas vent in question exists in the "major" diameter of the counter bore. The bolt rings are virtually a perfect seal. Until the bolt unlocks and the bolt rings move past the front two gas ports, the only pressure relief is out the back of the terminus. Pressure fro the hammer keeps the firing pin flange closed against the back of the counter bore making the third vent hole the path of least resistance.

[TripleBravo]

There is actually a counter bore in the journal of the bolt carrier that supports the tail of the bolt. The "minor" diameter (section closest to the bolt rings) acts with the bolt tail to seal the back of the terminus. The gas vent in question exists in the "major" diameter of the counter bore. The bolt rings are virtually a perfect seal. Until the bolt unlocks and the bolt rings move past the front two gas ports, the only pressure relief is out the back of the terminus. Pressure fro the hammer keeps the firing pin flange closed against the back of the counter bore making the third vent hole the path of least resistance.

Great explaination...thanks, Mark.

[Bad Hammer]

There is actually a counter bore in the journal of the bolt carrier that supports the tail of the bolt. The "minor" diameter (section closest to the bolt rings) acts with the bolt tail to seal the back of the terminus. The gas vent in question exists in the "major" diameter of the counter bore. The bolt rings are virtually a perfect seal. Until the bolt unlocks and the bolt rings move past the front two gas ports, the only pressure relief is out the back of the terminus. Pressure fro the hammer keeps the firing pin flange closed against the back of the counter bore making the third vent hole the path of least resistance.

So, the design intent was to vent the "excess" gas before the gas actually performs it's primary function of cycling the action, and the designer
decided to do this by forcing hot gas at very high pressure through the tiny confines of a critical bearing surface...
I'd imagine that if this was how it happens that the bearing surface would erode at an alarming rate.

Edited to add the following...

In keeping with your " Pressure from the hammer keeps the firing pin flange closed against the back of the counter bore " analogy, which is absolutely correct,
I'd think that the "third hole" is more of an intake port to prevent any sort of vacuum locking that may hinder the carrier in it's cycling.
Kinda like the tiny hole in the bolt itself near the base of the extractor channel, to allow the extractor to move in the cylinder without any compression hindering it's function.

[M. Gale]

So, the design intent was to vent the "excess" gas before the gas actually performs it's primary function of cycling the action, and the designer
decided to do this by forcing hot gas at very high pressure through the tiny confines of a critical bearing surface...
I'd imagine that if this was how it happens that the bearing surface would erode at an alarming rate.

Edited to add the following...

In keeping with your " Pressure from the hammer keeps the firing pin flange closed against the back of the counter bore " analogy, which is absolutely correct,
I'd think that the "third hole" is more of an intake port to prevent any sort of vacuum locking that may hinder the carrier in it's cycling.
Kinda like the tiny hole in the bolt itself near the base of the extractor channel, to allow the extractor to move in the cylinder without any compression hindering it's function.

In hindsight, excess gas is a misnomer. Any gas that transits the counter bore is better vented out of the receiver than into the receiver.

There are some crazy things happening in the bolt group during the cycle of operation. I am not certain what the "designer" intended. I think we can all agree that many of the modifications made to the platform are generally a result of unintended consequences of the original design or a subsequent change.

I am not an engineer and certainly can't offer any science to support my belief, but after noodling the vacuum theory for awhile, I'm not sure its plausible. I will rattle Elmore's cage today and see if he has anything to add.

[TehLlama]

My assumption for the LMT carrier was that it allowed larger surface area for venting gas faster so it wound up outside the upper receiver (via ejection port) instead of caked against the inside of the upper near the FA. Anything else seems inaccurate or unlikely.

That third smaller hole is just where the rest of the firing pin retainer ends up - I wouldn't want a pin like that walking and dragging against a surface in the upper. Since it's behind the cylindrical block where the firing pin goes to its narrow diameter, I don't see how it's designed to move gas in any useful way, and doubt that machining the bolt carrier to allow gas to pass through there would be helpful.

[Bad Hammer]

My assumption for the LMT carrier was that it allowed larger surface area for venting gas faster so it wound up outside the upper receiver (via ejection port) instead of caked against the inside of the upper near the FA. Anything else seems inaccurate or unlikely.

That third smaller hole is just where the rest of the firing pin retainer ends up - I wouldn't want a pin like that walking and dragging against a surface in the upper. Since it's behind the cylindrical block where the firing pin goes to its narrow diameter, I don't see how it's designed to move gas in any useful way, and doubt that machining the bolt carrier to allow gas to pass through there would be helpful.

The third hole in question isn't the retaining pin hole, nor does it have anything to do with LMT's modified carrier.
The hole being discussed is located forward of the retaining pin, just at the beginning of the forward assist serrations.
It is present in the stock bolt carriers, as well as LMT's carrier, although LMT moved it up higher on the carrier for whatever reason.