Title says it all. Specifics of brand, etc is unimportant at this time. Presume that ammo, mags, and gun are in order.
Some things that will help…
Bolt locks to the rear on an empty mag, mag hardware is practically new.
Carrier is staked properly via MOACKS.
No gas leakage around gas block.
Bolt recedes in to carrier when set upright on a table. (Gas ring test?)
No insert on extractor spring, barrel is sub 14.5".
I believe this may be a combination of several factors related to an over gassed carbine. Assuming that everything in within spec, some variation of this cycle of events may be happening when everything lines up…
The location / size of the gas port is allowing the upper end of acceptable amount of gas to enter the system. This is causing a very violent unlocking of the bolt and rearward movement of the carrier… This force is being transferred to the top round in the magazine, through all the stacked rounds in the magazine, and onto the follower and magazine spring… The magazine spring is being compressed slightly under this force releasing the upward pressure on the rounds loaded into the magazine that are also being pulled down by gravity…
Whilst still under recoil, the magazine spring pushes the rounds back up; however the top round is now misaligned and it is being ejected out of the magazine at the same time the bolt carrier comes forward catching both the top loose round and the new top round in the magazine…
Then its “Shamwow”… Double feed…
This will not happen all the time as everything has to be “just right” and all the events have to happen at exactly the right time with the correct delta V, but this may be what is happening…
Just my .02
B
Would this be an example of magazine surge, where a stiffer mag spring might solve it? Instead of doing something about the over-gassing?
See if it happens when the same mag is fully loaded vs. nearly empty. In an overgassed setup, fully loaded mags with good springs will often spit out 2 rounds at a time until the first 8 to 10 rounds are gone. Start with a mag that has shown this problem while fully loaded and only put 3 or 4 rounds in it to see if it still does it. If it doesn’t do it with a nearly empty mag you can build up to see how many you can have in the mag before doubling occurs. This won’t fix the problem obviously but will help diagnose it. If your problem is overgassing you will have to deal with the gas issue, springs alone won’t be the answer.
If the double-feed is the one expended, one live case (as opposed to 2 live, which is almost always magazine related) I’d have to say it’s an extractor issue. Make sure the extractor claw is GTG and add a standard O-ring or Defender to make sure the extractor has sufficient tension on the case rim. Also I don’t think the bolt should slide back into the carrier when sat upright. Replace the gas rings.
If it’s 1 live and 1 spent, it’s an extractor issue.
2 live rounds is mag related.
There may be other variables too.
I’ve got a quick side question for you, SHIVAN. My bolt doesn’t do this, it’s actually kind of a semi-tight fit. Meaning that it needs a little bit of pressure applied to it before it recedes into the carrier. Both the BCG and bolt were manufactured by Colt, the BCG being factory (came with the rifle) and the bolt being rather new. Could this just be a product of the bolt being newer and perhaps not ‘broken in’ yet, or an issue with tight tolerances? For the record, I haven’t fired the weapon since installing the new bolt a couple days ago. For that reason I’m obviously not concerned about anything. I am, however, curious as its predecessor would recede into the carrier when sat upright. As has any other one I’ve ever been issued in the military or had as a civilian.
My personal experience with this was the weapon I was working on was over gassed and I replaced the buffer spring with a wolf extra power spring and added a heavier buffer and the problem was solved. It seemed that the bolt was moving faster that the mag spring. After using several different brand new mags, I ruled the mags out as the issue. Slowing the bolt down solved it.
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If the double feed is one live and one spent round, wouldn’t that be an Ejector issue? If it is trying to double feed, trying to feed two rounds at the same time, the spent round was successfully extracted, it just wasn’t ejected? …Correct?
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Shivan,
Isn’t the above what you meant??
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^^this.
Bolt recedes in to carrier when set upright on a table. (Gas ring test?)
Not good. Time for a fresh set of gas rings. Just for the sake of ruling things out, a detail strip/clean and rebuild of the bolt makes sense as well (new ejector spring, extractor spring w/insert, extractor if well worn).
Is this a gun that previously worked, and only recently started having problems?
agree with the posts above, the bolt is NOT supposed to recede into the carrier in that test.
A double feed does NOT involve a spent cartridge, period.
Double feed = bolt is trying to feed two live rounds.
Is this suppressed or unsuppressed?
What type of stock are you using and what buffer spring and buffer?
My recommendation is to change the equation and try another known good BCG (check for excessive headspace with a 5.56mm FIELD or FIELD II gauge before firing).
This is true. However it is a very common misconception/mis-statement.
A true double-feed of two live rounds is almost 100% a magazine issue (IME).
A failure to extract in which a live round tries to load and the casing is still inside is more than likely an ejector issue. I will confess that practicing AR medicine over the net is very difficult.
Bolt and Carrier are older, but not high rounds. Bolt recedes in to carrier under it’s own weight. Yes, aware of nomenclature, this weapon tries to feed two live rounds.
It has also done the spent case, live round tango too. Unfortunately. In one instance, it was two live rounds and one spent case. :bad: Figure a new o-ring and extractor spring will cure that ailment though.
Mag hardware is practically new. Happens irrespective of mag. All ammo is FMJ. Unsuppressed. Heavy buffer; carbine (5.5oz), standard spring.
It’s hard to diagnose over the net.
If an empty casing is not leaving the bolt and ejecting out of the upper then you may have a bad ejector, bad ejector spring and debris/carbon down in there keeping the ejector from working correctly.
Two live rounds is the mag, or the mag catch isn’t adjusted properly (button should be flush with the mag catch threads).
It’s pretty safe to assume that you’re getting plenty of gas since you said it locks back on an empty mag.
[i]When I work on problem rifles I check a whole gamut of things and one thing I always do is remove the carrier key, degrease the carrier and key, lap the key on a Arkansas stone, degrease again, apply Rocksett to the base of the key, install two new screws, torque to 50-55 in lbs and stake with a MOACKs. I’ve found many a tight carrier key to be leaking. You’ll see the carbon tracking after the key is removed.
A few weeks ago I repaired a friends 11.5" Sabre Defence SBR which had problems. His was a bit complicated. It took a few trips to the range to try different things and figure out what was going on. His SBR had multiple things going on. Originally it was set up with a MGI buffer and Surefire 212 brake. It would short stroke not locking back when empty and sometimes not feed the next round. With my BCM carrier from one of my rifles it wouldn’t short stroke anymore but the BCG would out run the magazine and not strip off and feed the next round (it would completely miss the live round) bolt closing on an empty chamber when using a H buffer. That’s when I pulled the FSB and noticed it was way lose and that the gas port was too big. Installed a new VLTOR gas block and tried again with the Sabre carrier and H2 buffer, still problems but worked somewhat better, tried the BCM carrier (from my 3gun rifle) which ran much better but still not 100% with an H2 buffer (would lock back 80% of the time). Tried and H3 buffer and then everything worked 100%.
Problems found:
Too large of a gas port in the barrel .089" (11.5" bbl)
Too lose of a FSB to barrel fit
Bad bolt carrier (not sure what’s up with it)
Fix:
New BCM carrier (he opted for a whole new BCM M16 BCG, keeping his Sabre bolt as a spare)
New BCM H3 buffer
VLTOR low pro gas block (installed and pinned in place)
Rifle runs 100% with all mags tested (Colt, Magpul, Lancer, Tangodown)
[/i]
If the bolt collapses under it’s weight, then yes you do need new rings. If the gas system is wonky, then that may well be PART of the issue.