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Posted: 2/24/2017 11:53:29 PM EDT
There are actually 4, the one furthest to the rear being the hole for the firing pin retaining pin.
The next one is between the hole for the firing pin retaining pin and the section of the bolt carrier that was cut away for the spring detent on the dust cover to fit. This hole appears to line up with were the rear of the bolt. What is the purpose for this hole? The last 2 are 2 vertical holes that are centered on the section of bolt carrier were the spring detent for the dust cover fits when the dust cover is closed. These 2 holes are just forward of the gas rings with the bolt pressed completely in on the bolt carrier. What is the purpose of these 2 holes? Thanks in advance for the help. 
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Posted: 2/24/2017 11:56:18 PM EDT
[#1]
Gas vents
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Posted: 2/24/2017 11:56:48 PM EDT
[#2]
Don't know but I use them as lube holes.
Vince |
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Posted: 2/25/2017 12:01:30 AM EDT
[#3]
Quote HistoryQuoted:
Gas vents |
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Posted: 2/25/2017 12:17:14 AM EDT
[#4]
You squirt your lube in there.
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Posted: 2/25/2017 12:46:28 AM EDT
[#5]
They are gas vents. They are located where they are an open path for the gas to escape once the carrier has moved rearward enough to have unlocked the bolt. The vents are closed while the bolt is locked.
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Posted: 2/25/2017 1:05:44 AM EDT
[#6]
Quote HistoryQuoted:
They are gas vents. They are located where they are an open path for the gas to escape once the carrier has moved rearward enough to have unlocked the bolt. The vents are closed while the bolt is locked. Yup, but they are also good places to insert a couple drops of oil without field stripping. |
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Posted: 2/25/2017 1:06:05 AM EDT
[#7]
Gas vents - which dump pressure from inside the bolt carrier out the holes and also right out the ejection port.
The gas used to power the action can be seen exiting the gun in high speed photography just before the case starts to tilt out for ejection. Most of the gas residue left in the action of the M16 passes over the exiting brass from the chamber. Its why the brass gets dirty compared to a manual bolt action, same as any other blowback .22 or .380. Those of us who have owned HK91's understand this fact - its one of the more notorious guns for getting dirty, but it has NO gas action at all. It's roller delayed blowback, all the gas residue exits the chamber into the action. Some BCG makers have three holes at that location. |
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Posted: 2/25/2017 7:38:46 AM EDT
[#8]
Quote HistoryQuoted:
Yup, but they are also good places to insert a couple drops of oil without field stripping. Yup, I always put some lube in there after cleaning/reassembling the bolt. Then I work the bolt in/out of the carrier. Wet, wet, wet. |
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Posted: 2/25/2017 9:03:31 AM EDT
[#9]
Quote HistoryQuoted:
They are gas vents. They are located where they are an open path for the gas to escape once the carrier has moved rearward enough to have unlocked the bolt. The vents are closed while the bolt is locked. +1 Look at the gas vents holes in this slow motion video Inside the M4 carbine by LAV |
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Posted: 2/25/2017 10:27:40 AM EDT
[#10]
Quote HistoryQuoted:
Gas vents |
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Posted: 2/28/2017 2:53:43 AM EDT
[#11]
Quote HistoryQuote HistoryQuoted:
Quoted:
They are gas vents. They are located where they are an open path for the gas to escape once the carrier has moved rearward enough to have unlocked the bolt. The vents are closed while the bolt is locked. +1 Look at the gas vents holes in this slow motion video Inside the M4 carbine by LAV That was a kool video. 
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Posted: 2/28/2017 3:08:13 AM EDT
[#12]
Might take a gander at this thread:
http://www.ar15.com/forums/t_2_130/165511_How-the-Piston-in-your-DI-AR-works.html The two holes in the indent are exhaust vents from the gas cylinder. If you look through the holes as you manually retract the bolt carrier, you'll see the gas rings in the bolt go past the holes. Gas goes into the cylinder, pushes the bolt carrier back, when the rings go past those holes the gas is vented out from the gas system and out the ejection port, inertia of the moving bolt carrier does the rest. |
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Posted: 2/28/2017 9:04:13 AM EDT
[#13]
Cool video. It always amazes me how much the barrel flops around like a water hose.
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Posted: 3/1/2017 3:24:29 AM EDT
[#14]
Yep. The AR is much like a motor. You have the gas coming into the gas key (much like an "intake") the pressure builds up rapidly against the gas rings (much like a piston), and as the bolt and carrier separate, the rings pass the vent holes (much like exhaust ports) to relieve excess pressure.
The AR is more "motor" like than any other system I'm aware of. Pure Genius. That's also why I use motor oil in my AR.... but that's a different (heated) topic 
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Posted: 3/1/2017 3:54:39 AM EDT
[#15]
Quote HistoryQuoted:
Yep. The AR is much like a motor. You have the gas coming into the gas key (much like an "intake") the pressure builds up rapidly against the gas rings (much like a piston), and as the bolt and carrier separate, the rings pass the vent holes (much like exhaust ports) to relieve excess pressure. The AR is more "motor" like than any other system I'm aware of. Pure Genius. That's also why I use motor oil in my AR.... but that's a different (heated) topic I have described the AR15 as an external combustion engine before, that's really how it works. |
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