Tuesday, March 2, 2021

The Bolt Carrier Group

Greetings Me Droogs N Droogettes!
As per requested, I'ma doing a bit o'breakdown today on the Bolt Carrier Group for the AR/M-4/M-16 series of rifles.
Two Main Types.
M-16 and AR-15 BCG
Main difference is about a 1/2 inch of steel shaved off the ass end of an AR.  This is to prevent the AR from being able to lock the M-16 Hammer into the sear that allows full on rock n'roll.  More on lower parts in another episode.
Now, as seen, the parts to the BCG include the Carrier that the bolt rides in, the cam pin that holds the bolt in place, the gas key which allows the bolt to operate, and the lil itty-bitty retaining pin that holds the firing pin in place as well.  That pin is best known as "that fuckin' pin" or "where the fuck did it go?".

Yah... teeny pin, but VERY important.  Holds the whole thing together.

So, The principle of operation:
Bullet goes >boom<, gas goes behind, gets diverted into the gas tube and shoots a squirt of gas into the gas key.  Gas key is pushed back and vents gas into the rear of the bolt causing the whole bolt itself to cycle to allow the bolt to unlock from the cams and wash, rinse repeat.

I mean there's a -bit-  more in that, but if you want to  be super nit-picky, great... go ahead but trolls sayin' "That's not right" are going to be ban-hammered as this's my house, and when Babe Ruth is at the plate, one does not dare to pick up a baseball bat Aye?
OK... Pieces parts.

Rule #1 of owning a Combat Rifle: Have Spares.  One is None, Two is Good, MOR is Better.

I keep, as a rule
5 Spare Firing Pins, 2 are Titanium, 3 Steel
3 SETS of gas rings.  Meaning 9 individual rings.  Each bolt needs 3 to safely operate.
1 Spare COMPLETE Bolt AND Carrier Group, tested and headspaced.
2 Spare Gas Tubes for each 'flavor' rifle... in my case, 2 Carbine Length, 2 Midsized.
2 Spare Gas Keys with Bolts
2 Spare Extractors with springs
And Pin... Oh my fucking God the pins.  Try when you don't count 'em individually, but by the ounce... like a quarter pound of spare pins, springs and fiddly bits.

I got a couple of Harbor Freight lil pin-and-screw boxes to segregate and separate them into their proper families...  Get the good ones if you can, as you do not want to have one of these boxes pop open if the cat knocks it off the workbench.  Ask me how I know...

Also, the extractor pin on the face of the bolt. Don't fuck with it.  We had a very specific tool we used to replace that, and I don't think outside of training I ever fucked with that.  Leave it alone.

Now, lots have emailed and asked what the most common point-of-failure I've seen while working on these rifles over the years.  Whelp, the most 'stuff' I've seen is pretty usual for DotMil weapons, i.e. weapons that're really old and rebuilt hunnerds of times... shit that have been in multiple wars...

Most common issue(s) are interconnected.
Worn/Mushroomed Gas Key and Gas Tubes.  Gas keys and tubes that're worn out are noticeable.  The light aluminum of the tube gets worn after a couple thousand rounds...
Gas tube on top is worn but serviceable
Gas tube on bottom?  Worn smooth, needs replacing.
Mushrooming, (and I can't find a good picture) is when usually the Gas key is slightly mis-aligned with the gas tube, usually because some over-eager Private jammed a cleaning rag in between the gas tube on the interior of the upper receiver, bending it just enough that the face of the gas key starts getting beaten by and on the tube.  It is steel, but with a really fine edge on it that if misaligned can fuck it up.  That and boneheads who drop the fucking BCG can damage it as well...
Another issue:
Can you see it?
No staked Gas Key.
It should look like this:
IF you have to re-do a gas key, make sure you have the tool for it.  Do not beat the fuck out of your BCG with a screwdriver and a mallet.  Bubba shit right there Aye?
Now some folks go "Well... fine, I'll have a few things but why so many?"
Because you never rely on the supply chain.  Ask how those gomers who didn't have anything more than a hunnerd rounds on hand for their primary AND secondary total when the cost of Boolits went from $.20 cents a round to over $2 dollars a round and those who didn't have at least 1000 primers onhand when they too disappeared.

Case in point, 7 years ago, plain as day... I had my first firing pin break ever during a casual range day.  Fired 3 rounds out of 10, and then no more bang-bang.  Check the round 3 times, and finally was like, "no dimple on the primer... hmmn"  Took the bolt apart and sure as fuck, the verrrrry tip of the firing pin had sheared off.  First time I've ever had that happen.  And the bitch?

I didn't have a spare with me.
So yeah, spares = good
No spares = bad
Keep it in mind.
Especially since they appear to be lining up to outlaw the sale/purchase of spares.
Get 'em while you can
Before you Can't
More Later I Remain The Intrepid Reporter
Big Country


  1. Word of caution: The rifle will operate with one gas ring on the bolt.

    Also, do not worry about gas ring location, they float and rotate on their own, and carefully spacing the gaps will amount to nothing, because they will move at the first round fired.

    Lastly, the rifle does not need to be spotlessly clean to run. It does need to be lubricated, the wetter the better. Anything will work, for a time, but some lubes will stay in place longer than others. I use SLIP 2000 EWL. If you don't think a AR pattern rifle can run without cleaning, google up the SWAT magazine article on EAG Rack # 14, AKA "Filthy 14." 35 plus thousand rounds without cleaning. Rack #14 was a loaner rifle for students at EAG carbine class, which was a 3 day class with a round count into the low thousands. It made it into the low forty thousand rounds fired without cleaning before it was retired.

  2. Excellent advice/information !

  3. In the bolt face diagram, isn't that the ejector, not the extractor spring?

  4. Good stuff, thank you for this.

  5. Spare parts for AR rifles used to be cheap and plentiful. That fact.....like many.... Is changing. Get what you can while you can. Just like a lot of other necessary items are going to be made scarce either by demand or by decree.

  6. While I like my SR-556 it does have a couple draw backs, one being the gas key is non-standard because it is piston driven not direct gas. Having a hard time finding anywhere that has them available.

  7. Care to comment on one piece gas rings?


    I have been using them for a long time and like them. Yes, you can make an argument that they are a solution looking for a problem. Still, I like them.

  8. Umm... the gas tubes are made of stainless steel or if you want a Gucci one, inconel.

    Not aluminum.

    Aluminum of that thickness cannot handle the pressure.

  9. I posted something this evening and then looked at my blog roll and lo and behold you posted a much more comprehensive variation. Excellent info.

  10. Good stuff. Most people barely have enough ammo; dear God what a trial I've had trying to convince people in past years "no, buy TWO cases now, not just half of one..." and "no, you should get that 10-pack mag deal now, not just two overpriced mags at your local shop".
    Now consider that spare parts are even less on people's radar than mags/ammo. Ugh...

    Anyway, useful contribution time per gas rings:
    Once upon a time a certain newb idiot who we shall call "Anonymous", fresh from purchasing their first serious-quality riffle, was at a shooting event. Early spring, snow, mud, good times. Dropped a mag trying to do a running reload between cover positions. Picked it back up, saw there was a nice chunk of mud in the boolits end of the mag. This nameless "Anon" then decided "WTF? I have a high speed gun...let's see if it can handle a little mud like the torture tests say they can."

    Oh dear readers of the comment section... be ye not tempted to walk in the foolish ways of Anon, for though shalt immediately be smitten with totally preventable malfunctions.

    Namely, a round conveniently stuck SOLID in the chamber via mud injection.
    Immediate record-scratch to gun-no-worky, hard stop, do not pass go or collect $200. #bigOof
    Anyway, had to mortar the *shit* out of riffle (extra fun in soft, muddy ground, makes this technique super-not-effective).

    Later the next day was cleaning the gun; upon disassembly of BCG, broken gas ring just fell out. Found second cracked ring upon further inspection. Was extremely shocked as rifle ran perfectly for the rest of the event. A certain Anon did not have spare gas rings at that time, but by God they had a high-speed rifle...
    Needless to say, certain lessons only need to be learned once.

    Takeaways for fellow readers:
    > Don't put chunks of mud in your AR.
    > Rifle can totally work with busted gas rings which you may NEVER notice until cleaning/inspection.
    Mine ran like a top for around 100 more rounds that day, with 1 totally busted ring, 1 cracked ring, AND a light residue of mud remaining in the chamber (from the voluntary mud insertion, then using like, a less-muddy stick to poke the worst of the mud-clump out).
    > I suspect some of the continued running was due to correctly spec'd/chromed 5.56 chamber and Slip 2000 EWL
    > Clean/inspect your rifle after every shoot to find dumb problems like this before they become, in the parlance of the current year...problematic. Have spare parts to fix the problems.

  11. Can't wait for the headspace block of instruction !!

  12. If the 3 gas rings all align spontaneously, you can get FTFe/FTFi/FTE malfunctions.
    Though slightly harder to install, the single-piece gas rings make that potential go away.
    Dealer's choice. Either way, have spares.

    And it's hard to go wrong if you start grabbing and collecting 2/3/half a dozen blister-packaged "lower receiver parts kits" from the better makers, just to squirrel away for what-if.
    Grab the other bits and bobs for the upper as well, and you're good to go.
    The proper tools and jigs for assembly/maintenance are the difference between actual "repair", and "Bubbaing".
    Don't be a Bubba.
    Repair isn't hard; Big Green teaches h.s. dropouts with GEDs and a modicum of aptitude how to do this.
    SF teaches 6th grade dropouts from Shitholia and Trashcanistan how to do this.
    Get the parts, get the right tools for the job(s), and download the manuals (and print them out hard copy), and you're good to go, and can run one indefinitely.

    Last I heard, Mountain Guerrilla was running his with plain old engine oil (can't recall whether he preferred actual oil, or synthetic). Anything that will keep a Toyota running at 4000-5000 RPMs for 15K miles isn't going to have trouble dealing with something cycling 100-600rpm from Mr. Stoner.
    But engine oil isn't cleaning fluid, which it will need from time to time, depending most especially on how craptastic dirty your powder is. Actual military stuff, traditionally, is pure dirty gunk. Commercial spec ammunition tends to be cleaner. People in humid coastal lands will generally want a slightly different lube regimen than those in hot, dry sandy climates.

    Have spares. Murphy eats gun parts, in inverse relation to how critical immediate function is at the time.
    You can also fit a small crafting plastic ziplock bag with the most vital bolt/BCG spare parts, pins, and springs inside one of those trapdoor pistol grips, and solve a lot of troubles on the spot.

  13. Roger!

    They is called "WHOOPS!" kits. Filled with all flavors of goodies.



    git sum

  14. great technical instruction Bro, this is some of the best i've seen yet

    300 million..