Gunsmith quiz 2

[wichaka]

Had a neighboring LE agency bring me a new Bushmaster, said it jammed hard after 10 rounds.

Pulled it apart. which took quite a doing...and this is what I found;





After doing some checks I figured out what caused it.

Anyone wanna take a stab of what the cause was?

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[The B]

burred vents in the carrier?

 

[MarkSBG]

Was the bolt ring bent open during installation?

 

[coctailer]

Gun fired without cam-pin installed.

 

[The B]

Gun fired without cam-pin installed.

they got 10 rounds off before the failure.. the bolt wouldnt leave battery without a cam pin- at least not 10 times

not even once, 99 times out of 100

 

[wichaka]

burred vents in the carrier?

That was my first thought, but nope...everything is smooth in the bore.

 

[The B]

hmm.. new gun, blown rings... that bolt's got some odd wear, especially for only having 10 rounds on it. does that wear extend the entire circumfrence on the tail and on the ring?

 

[wichaka]

Yes, but not in the equation.

 

[wichaka]

Was the bolt ring bent open during installation?

Nope, as we tried it with new rings and it did the same thing again.

 

[The B]

how tight is the bolt inside the carrier?

and you say the carrier bore is smooth- no burrs, not unfinished, etc?

 

[tionico]

well, never having seen a gas-operated semi auto apart before, my guess is that the pictured piece is the gas piston, the rings seal it in its own bore. The large holes mount the op rod.

My guess.. something is haywire with the op rod, or the bolt itself (can't imagine headspace being that far off, but hey, its a piece of machinery that someone assembled...), allowing this piston to come too far out of its bore, and, when it returns, the trailing ring then gets caught up on the end of the bore, jamming.

give me a detailed photo tour of the whole mechanism, or put it in my hands, and I'll work out what went wrong.Too many missing bits, can't work out where the problem is seeing only the result. You remember the old hippie thing.. everything is connected to everything else.....

 

[wichaka]

Its a gas operated AR system, no piston.

What you see is the bolt that fits into the bolt carrier. The bolt shrouds the case head of the round.

The gas rings in question here, seal the gas from flowing forward in the bolt carrier group.

tionico
"......allowing this piston to come too far out of its bore, and, when it returns, the trailing ring then gets caught up on the end of the bore, jamming."

You're real warm with that one. Very close...

 

[aflineman]

If there are no burs, my second guess (without seeing the entire thing) is the bore is to small.
Third guess is the grove that the gas rings are in is not straight.

 

[The B]

all that's left is dimension.. either the bolt or carrier is out of spec, whatever the case. even if headspace was somehow so bad as to allow the bolt to ride forward enough for the rings to catch on the cam-pin slot, the cam pin itself keeps that from happening- unless the cam-pin slot is milled wrong, allowing the bolt to ride too far forward. in which case, the bolt would ride forward enough to catch the lip regardless of headspace, during extraction.

 

[tionico]

That much is obvious from looking at the sealing rings..... the last one is coming out of the bore it rides in by just enough to hang up.

But, without at least seeing a parts breakdown and how they work together, I can't guess the rest of the story. A mechanism I've never seen, even in a drawing. Hard to guess WHERE the tolerances are out in order to pinpoint the root cause. I can only guess how the damaged bit fits into and with the neighbouring bits.

so--without a schematic breakdown, I can go no farther. But, like I said, put ALL the bits into my hand and I'll get to it quickly.... AND the cure.


besides, what presently remains within my cranium has so greatly diminished, in both quantity and quality, as to render further pondering on this arcane mechanism rather pointless..... and so I shall desist for the nonce, and point said cranium toward by night's resting place......... if I can remain awake that far.

 

[The B]

it's actually the first ring.. and what's happening, if the carrier cam slot is indeed the problem, and it seems that it probably is, is that as the BCG extracts, the first ring is just barely clearing the opening, then immediately getting sandwiched/scissored in as it slams back home. and the photograph actually kind of provides a hint toward this- right where the break occures is exactly where it would happen. if i'd paid any attention to the position, i may have suspected it sooner... but probably not. plain old bad machining in parts like carriers and receivers and such is so rare that it's hardly ever the cause for problems.

so is that it, wichika?

 

[wichaka]

Ok, will end this here;

There are 3 bores in a the bolt carrier in which the bolt slides in to;

1) Which is the diameter of the main body, as can be seen in pic 1. There's a band that goes around the main body of the bolt, which is located just aft of the extractor pin and in front of the cam pin hole;

2) The gas ring chamber;

3) The tail shaft.

There "should" be a gradual shoulder from the main body bore to the gas ring chamber, so the rings will go into the chamber with some pressure but without causing the rings to jam...in this case there wasn't, it was very abrupt in some areas.

1) That abrupt edge, which varied in sharpness caught the first gas ring once the ring spun around to one of the many sharp edges.

2) The main bore was machined a tad long, allowing the first ring to come out of the gas chamber hole far enough to expand, then get caught on the abrupt shoulder.

Contrary to popular belief, it does not matter if the gaps in the gas rings are aligned. With firing, the rings will rotate around anyway and the gun will still function normally.

 

[coctailer]

Aaahhh