View Full Version : Short stroking BCM bolt carrier
acaixguard
09-04-21, 19:49
I have a BCM which I shot today, and suddenly ran into it short stroking. Filmed a friend shooting my rifle in slo-mo, and the bolt only cycles half of the stroke. Basically, it doesn't even go back enough to eject the empty case, so it goes right back in the chamber.
Fortunately, one of the guys I shot with had a spare rifle, so he let me take use it to run some diagnostics. First, I used my upper on his lower, and the same thing occurred. Thus, I eliminated the buffer and buffer spring as the problem. Then I used his entire BCG in my upper. It then cycled perfectly fine. So now we know it's something within the BCG, and not because of the gas tube being undergassed, obstructed, or out of position somehow.
Did a little more testing, and used his bolt in my bolt carrier. The short stroking occurred again. Then I used my bolt with his bolt carrier, and voila, the gun cycled great once again!
So now I'm pretty sure there is something wrong with my BCM bolt carrier. If I had to guess, it must be something with the gas key. I last used the rifle in June for a 2 day class, and it ran like a champ then. The only thing I have done since then was clean the rifle entirely, and kept it lubed. When I cleaned it last, I ran a pipe cleaner through the gas key, so I don't think it could have been obstructed. But who knows if a piece of the pipe cleaner broke off, or some other crud got lodged in there.
Are there any other suggestions to rectifying the problem? I cleaned my BCG again tonight, and did all I could to make sure the gas key was free flowing, shy of removing the gas key from the carrier. Or should I contact BCM about what they can do about it?
Thanks in advance!
Try a reverse torque test on the carrier key screws? I think 30 inch-pounds was recommended.
Try a reverse torque test on the carrier key screws? I think 30 inch-pounds was recommended.
Before I tried reversing, I would try and tighten them to 50 inch pounds and see if the screws move.
If its not your only rifle and you can stand having it inoperable for a bit, they've had great CS in my opinion so I'd just email them everything you posted here.
Before I tried reversing, I would try and tighten them to 50 inch pounds and see if the screws move.
SOTAR does 20 in-lbs, then 30 in-lbs, both reverse. He said he never exceeds 30 inch pounds for testing. Ten minutes in on this video.
https://www.youtube.com/watch?v=OJ0gWT6Hx9w
SOTAR does 20 in-lbs, then 30 in-lbs, both reverse. He said he never exceeds 30 inch pounds for testing. Ten minutes in on this video.
https://www.youtube.com/watch?v=OJ0gWT6Hx9w
There is no reason to test whether or not BCM correctly tightened the carrier screws, they did. The carrier almost certainly has a loose (unlikely) or broken carrier screw. Tightening them is the correct troubleshooting step in either case, but why?
As Wake said, the best action is to contact BCM.
Andy
Well, it was good thinking to do the parts change with your shooting friend, at least that way you were able to narrow it down to the carrier. Personally, I would do what an earlier poster said, and see if BCM can look at it to see what they think...who knows, you might get an entirely new one out of the deal.
acaixguard
09-04-21, 21:02
Ok, I tried the above suggestions. The rear screw was snug at 30 in-lbs, but the front popped out of position as I was approaching the same torque. Strangely, now the screw spins freely. It doesn't come up off the gas key when turning counterclockwise, and when trying to tighten again, it just spins infinitely. Possibly stripped screws/threads? Would I be able to send the bolt carrier to BCM for fixing/replacing?
Thanks again for the help!
Most certainly stripped threads.
acaixguard
09-04-21, 21:07
Thanks! Very strange...just something that happened during production? I have no more than 1k rounds through it, and never touched the screws till now.
Anyhow, I will be contacting BCM after the long weekend. Hopefully they can help me out.
Thanks! Very strange...just something that happened during production? I have no more than 1k rounds through it, and never touched the screws till now.
Anyhow, I will be contacting BCM after the long weekend. Hopefully they can help me out.
I would guess the screw is broken not stripped. Its a not-uncommon failure, although not normal at 1000 rounds. BCM certainly should help you.
Andy
Strangely, now the screw spins freely. It doesn't come up off the gas key when turning counterclockwise, and when trying to tighten again, it just spins infinitely. Possibly stripped screws/threads?
This is just like the video I linked above, and there it was a sheared screw. Send it in and get it fixed.
If is your only AR then I would buy a spare BCG so that way you can change over to your spare BCG and worry about it later.
acaixguard
09-05-21, 16:09
If is your only AR then I would buy a spare BCG so that way you can change over to your spare BCG and worry about it later.
I've been looking but it doesn't seem like any quality mil-spec ones are in stock?
Brownells M16 nitride BCG (http://www.brownells.com/rifle-parts/bolt-parts/bolt-carrier-groups/m16-bolt-carrier-group-5-56x45mm-nitride-mp-sku078102001-105377-202837.aspx) is on sale for $100. A good spare for the price.
There is also the Expo Arms mil-spec carrier (https://www.primaryarms.com/expo-arms-5.56-nato-complete-mil-spec-m16-bolt-carrier-group-phosphate) for the same price. Primary Arms house brand I believe. Probably not crap.
Both have a lifetime warranty.
ToolCraft shows in stock:
https://www.armorally.com/shop/toolcraft_5-56_nato_black_nitride_bolt_carrier_group/
sidewaysil80
09-05-21, 20:19
I guess whoever built this one didn’t get the seabag speech.
I've been looking but it doesn't seem like any quality mil-spec ones are in stock?
I'd look at Black River Tactical:
https://blackrivertactical.com/WP/BCG-Upper-Parts-c1852003
Also available at Forward Controls Design's website:
https://www.forwardcontrolsdesign.com/brt-m4a1-phosphate.html
Clint and Duffy (owners of BRT and FCD, respectively) are solid guys and contributors on this forum.
I would guess the screw is broken not stripped.
Andy
Same thoughts here.
Did it start to get tight and then spin, or did it just spin. Seems odd that it would have worked for the first thousand rounds if it came from the factory that way….
SOTAR does 20 in-lbs, then 30 in-lbs, both reverse. He said he never exceeds 30 inch pounds for testing. Ten minutes in on this video.
https://www.youtube.com/watch?v=OJ0gWT6Hx9w
Reverse torque check is not an acceptable inspection for final assembly (unless specifically called for by the tech data). Any fastener subjected to a reverse torque check should be loosened, the threads cleaned, inspected and re-torqued according to spec. A reverse torque can loosen the fastener without any perceptible movement leaving the fastener, not only under torqued, but loosened. Also, most torque wrenches are not certified for torque in a counter clockwise direction.
If there is the least suspicion a gas key is leaking, remove it. I've had a gas key fasteners that passed the torque check, where the fasteners were torqued in a clockwise direction with no signs of movement. Yet when the key removed, there were signs of leakage between it and the carrier. Cleaning and reinstalling the gas key solved the malfunction the AR was having.
Labor Day sale until Tue at Noon
https://www.surplusammo.com/products/saa-ar-15-m16-bcg-mpi-nitride-223-5-56-300aac-complete-bolt-carrier-group.html
SAA AR-15/M16 - BCG MPI -NITRIDE - .223/5.56/300AAC Complete Bolt Carrier Group $79.99
For EMERGENCY
SAA AR-15/M16 Bolt Assembly w/ Pins - Nitride 9310 MPI - 223/5.56/300AAC $27.99
If is your only AR then I would buy a spare BCG so that way you can change over to your spare BCG and worry about it later.
Yes, a BCG (or a couple) is one of those parts one should keep on hand.
Reverse torque check is not an acceptable inspection for final assembly (unless specifically called for by the tech data). Any fastener subjected to a reverse torque check should be loosened, the threads cleaned, inspected and re-torqued according to spec. A reverse torque can loosen the fastener without any perceptible movement leaving the fastener, not only under torqued, but loosened. Also, most torque wrenches are not certified for torque in a counter clockwise direction.
I am sorry if I spread an unsafe method. I have no training in these things but I assume Chad Albrecht of SOTAR does. Perhaps I have misunderstood his method or where he employs it.
I am sorry if I spread an unsafe method. I have no training in these things but I assume Chad Albrecht of SOTAR does. Perhaps I have misunderstood his method or where he employs it.
It is entirely possible that Chad removes, cleans and re-torques after his "reverse torque" tests. Everything does not make it to video.
EDIT - most torque wrenches are more accurate in the clockwise direction. Fastener torque measurements are more accurate when tightening. I am guessing in Chad's case an approximate reading is sufficient and the torque wrench adds a bit of uniformity.
Chad is not performing final inspection after assembly in his videos, he is effectively performing a "post mortem" to identify potential issues.
Personally, I would check for loose or broken fasteners by attempting to tighten them at the correct torque because I am too lazy to disassemble, clean, and then re-torque if they were tight in the first place. If they are loose or broken, it will be obvious either way.
Andy
So what happened? Stripped threads or a broken off fastener head? Either way, my bet is they over torqued it at the factory, the fasteners were sub par, or both.
this is an informative thread. good-to-know stuff. just-in-case.
I've been looking but it doesn't seem like any quality mil-spec ones are in stock?
BRT In Stock
https://d2j6dbq0eux0bg.cloudfront.net/images/812059/1643559957.jpg
I’ve been really impressed with the AO Precision BCG that came with my URGI upper. Especially for the price they can be had at individually.
I do wonder whether BCM will take care of you or just say it was your fault. Even if not, you’ll have a good spare bolt which is good to have.
It is entirely possible that Chad removes, cleans and re-torques after his "reverse torque" tests. Everything does not make it to video.
MW is correct, as usual; most torque wrenches are designed to be accurate in one direction only. Fastener torque is definitely only measured when tightening. I am guessing in Chad's case an approximate reading is sufficient and the torque wrench adds a bit of uniformity.
Chad is not performing final inspection after assembly in his videos, he is effectively performing a "post mortem" to identify potential issues.
Chad clearly distinguishes between autopsy and physical. He has stated that he does not reuse gas keys. I do not believe he is replacing gas keys on every carrier that gets a reverse torque test as part of the physical. He uses a Wheeler's F.A.T. wrench at 30 inch-pounds. https://youtu.be/nQWGQuLyUCU?t=1035
The military standard is that the stakes should not distort the side surfaces, and they should result in a removal torque no less than 55 and no more than 100 inch-lbs.
If this is correct the fasteners should require at least 55 inch-pounds to break the staking. Perhaps 30 is not enough to loosen properly fastened gas key screws, but it is enough to detect sheared ones?
Chad clearly distinguishes between autopsy and physical. He has stated that he does not reuse gas keys. I do not believe he is replacing gas keys on every carrier that gets a reverse torque test as part of the physical. He uses a Wheeler's F.A.T. wrench at 30 inch-pounds. https://youtu.be/nQWGQuLyUCU?t=1035
If this is correct the fasteners should require at least 55 inch-pounds to break the staking. Perhaps 30 is not enough to loosen properly fastened gas key screws, but it is enough to detect sheared ones?
I edited my post for accuracy. It is acceptable to measure torque when loosening a fastener. The value will be approximate, but that is certainly sufficient to verify gas key screws are correctly torqued.
Andy
I see lots of photos posted where people show keys that are supposedly improperly staked.
The military standard is that the stakes should not distort the side surfaces, and they should result in a removal torque no less than 55 and no more than 100 inch-lbs.
I would bet that many of the "improperly staked" keys are in spec of 55-100 inch-lb removal torque, and many of the home stake-jobs are over 100 inch-lbs removal, and hence out of spec.
So before you re-stake your carrier, perhaps check to see if you can remove the screw with less than 55 inch-lbs of torque. If you cannot, then it is in spec.
This method is a "destructive" inspection designed to check the process, not the individual piece. You will note there is a minimum torque and a maximum torque. It cannot be known what maximum torque will break loose the fastener unit that torque is exceeded and the fastener has broken loose.
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