View Full Version : Short stroking caused by BCG
Captains1911
12-16-20, 21:11
I have a new 14.5” mid length upper that won’t lock back on the last round with a new Toolcraft BCG. I’ve tested it with multiple ammo types, mags, buffer weights, buffer springs, and BCGs, and concluded that the Toolcraft BCG is indeed the culprit. However, the carrier key doesn’t look or feel loose and is staked properly, and the bolt gas rings appear in good order and fit snugly inside the carrier. I’ve never had this issue before and am just curious on what specifically about the BCG could be creating this issue.
dylank0723
12-16-20, 21:16
I had a similar issue with a triarc not long ago, narrowed it down to the bolt itself, I replaced the extractor and ejector springs and it still short stroked, swapped it with a colt bolt and it’s ran like a top ever since from 77gr all the way to wolf, not sure exactly what my issue was either
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Does the bolt move freely in the carrier? Does the BCG move freely in the receiver?
Captains1911
12-16-20, 22:25
Does the bolt move freely in the carrier? Does the BCG move freely in the receiver?
Yes, everything feels normal in that regard. I should also note that it’s a NiB coated carrier, and well lubed.
Do you have another AR you can try the suspect BCG in?
This can occur with barrels that are very conservatively gassed or setup for mixed suppressed use.
The actual issue is the BCG is out of spec, usually in the bolt tail interface or gas key bore.
This causes extra gas leakage and reduced cycling power.
Non mil-spec coatings are a yellow flag in this case and often contribute to improper finished dimensions.
"Value"' priced commercial only BCG suppliers tend not to do a good job holding these critical dimensions with any sort of regularity.
The same leaky BCG may tend to run without apparent issue in a common overgassed barrel. This doesnt mean all is good, but a case of one issue masking another.
Swap the BCG for a high quality actual mil-spec unit and the upper will likely run fine.
This can occur with barrels that are very conservatively gassed or setup for mixed suppressed use.
The actual issue is the BCG is out of spec, usually in the bolt tail interface or gas key bore.
This causes extra gas leakage and reduced cycling power.
Non mil-spec coatings are a yellow flag in this case and often contribute to improper finished dimensions.
"Value"' priced commercial only BCG suppliers tend not to do a good job holding these critical dimensions with any sort of regularity.
The same leaky BCG may tend to run without apparent issue in a common overgassed barrel. This doesnt mean all is good, but a case of one issue masking another.
Swap the BCG for a high quality actual mil-spec unit and the upper will likely run fine.
I had this exact issue with a buddy. His S&W Sport was magically cured when the bcg was replaced. He tried new extractor springs, gas rings, etc.
I helped a buddy fix a rifle recently that had this problem. In his case it was the carrier key. It didn’t feel loose, but one of the screws tightened easily when tested lightly with a hex key. I replaced the key and screws and used sealant and it ran.
Captains1911
12-17-20, 07:45
Do you have another AR you can try the suspect BCG in?
Yes it ran fine in one other AR that I tested it in, but that AR is just gassed higher, so it doesn’t really prove anything. The fact that another BCG cycles properly in the 14.5” upper, is a sure sign that it’s the BCG.
Captains1911
12-17-20, 07:45
This can occur with barrels that are very conservatively gassed or setup for mixed suppressed use.
The actual issue is the BCG is out of spec, usually in the bolt tail interface or gas key bore.
This causes extra gas leakage and reduced cycling power.
Non mil-spec coatings are a yellow flag in this case and often contribute to improper finished dimensions.
"Value"' priced commercial only BCG suppliers tend not to do a good job holding these critical dimensions with any sort of regularity.
The same leaky BCG may tend to run without apparent issue in a common overgassed barrel. This doesnt mean all is good, but a case of one issue masking another.
Swap the BCG for a high quality actual mil-spec unit and the upper will likely run fine.
Makes sense, thanks for the info.
I get this on a 14.5 middy that runs 100% suppressed. Gassed lean without the can, but slightly overgassed with the can. So weird. Not a big enough problem to where I've researched it because it's an ACOG setup shot mostly prone/longer distances.
One thing I had happen before is excessive spring tension on the bolt catch.... either due to the detent hole being shallow, or a spring too long.
Yes it ran fine in one other AR that I tested it in, but that AR is just gassed higher, so it doesn’t really prove anything. The fact that another BCG cycles properly in the 14.5” upper, is a sure sign that it’s the BCG.
Unless you know what the actual gas port size is, its a guess as to whether a gun is over gassed or not. But be that as it may, do you have another bcg you can try in the present troublesome gun?&
An aggressively staked key bolt may be under torqued or even broken in two without the bolt head or key appearing obviously loose.
Captains1911
12-17-20, 10:46
Unless you know what the actual gas port size is, its a guess as to whether a gun is over gassed or not. But be that as it may, do you have another bcg you can try in the present troublesome gun?&
Yes, a BCM BCG I borrowed from another rifle works reliably, locks back every time with the H1 buffer, sorry I should have made that more clear. I even tried the BCM BCG with an H3 buffer, and it locked back 1 out of 2 times, I would have tested this combination more but I was out of ammo at that point.
So the upper likes other bcg's and the bcg likes other uppers, but they won't play well together. If you have the the ability to stake gas keys, I would strip the key off, see whats under it, clean under it and remount it and stake it. That, or . just replace the key altogether. You're obviously leaking gas somewhere...maybe its like Clint said through the bolt tail.
Captains1911
12-17-20, 11:17
For what it's worth, this is the BCG in question. I was led to believe that these were regarded as decent quality, it's certainly wasn't inexpensive.
https://palmettostatearmory.com/toolcraft-logo-d-premium-5-56-nickel-boron-bcg-with-carpenter-158-bolt-5165449729.html
I started having short stroking problems on a high mileage suppressed 11.5" upper.
The BCG related discrepancies were as follows-
- worn gas rings. Installed new gas rings
- leaky gas key. Removed gas key. Cleaned mating surfaces, reinstalled key & staked
- gas tube worn where it fit into the gas key. Installed new gas tube
- gas tube worn where it fit into the gas key. Installed new gas tube
This was a problem on mid length gas tubes a while back. I know another member had this happen. I had to change out 2 of them.
Carbon was puking forward in the upper and into the handguard. Couldn't see it until I took the old gas tube out.
davidjinks
12-17-20, 19:00
1 of 2 problems, and I’m pretty sure they’ve been mentioned...
Gas key on BCG
OR
Under gassed gun and some tolerance stacking
I hope that the Toolcraft NiB bcg I stuck in my last build is gtg. (DD lw 16” middy). Haven’t run it yet. Wouldn’t have been my first choice but with parts availability being what it is, I went with it.
I’d love to get my paws on a REBCG but I’d likely win the lottery first, and I don’t play.
Captains1911
12-18-20, 06:36
1 of 2 problems, and I’m pretty sure they’ve been mentioned...
Gas key on BCG
OR
Under gassed gun and some tolerance stacking
Under gassed gun, even though it locked back with an H3 buffer and a different BCG?
Captains1911
12-18-20, 06:37
Delete
For anyone who needs an upgrade, we have a few of these in stock now.
https://d2j6dbq0eux0bg.cloudfront.net/images/812059/1643559957.jpg
ViniVidivici
12-19-20, 21:21
I hope that the Toolcraft NiB bcg I stuck in my last build is gtg. (DD lw 16” middy). Haven’t run it yet. Wouldn’t have been my first choice but with parts availability being what it is, I went with it.
I’d love to get my paws on a REBCG but I’d likely win the lottery first, and I don’t play.
It'll most likely be just fine.
davidjinks
12-20-20, 17:18
Anything is possible, that’s why I added the tolerance stacking part.
At this point it’s a SWAG without being able to actually shoot it myself.
Under gassed gun, even though it locked back with an H3 buffer and a different BCG?
I had a similar issue with a PSA 8” .300bo upper and a PSA premium phosphate BCG.
Cycling by hand I could feel the BCG carrier had more resistance in the upper than any other AR I own.
It would cycle supers but not subs unsuppressed.
Swapped it with the Toolcraft nitride BCG in a PSA 16” .300bo 16” upper. Now both run unsuppressed subs just fine.
Just tolerance stacking.
Averageman
12-25-20, 13:02
Tolerances stack.
A bit here and a bit there can make for a questionable/or even wrong result, move some pieces around and "There you go, everything works."
I used to work on Abrams Tanks, it was sometimes maddening,....
There are times when it makes sense and others when it enters the realm of voodoo.
For anyone who needs an upgrade, we have a few of these in stock now.
https://d2j6dbq0eux0bg.cloudfront.net/images/812059/1643559957.jpg
Curious if you guys are a 'commercial only BCG supplier', make your own BC's in house, and how long you guys have been at it. I did visit your website and the 'about us' info was big time lacking in details in my opinion.
"Value"' priced commercial only BCG suppliers tend not to do a good job holding these critical dimensions with any sort of regularity.
WHY would a BCG from BRT be an 'upgrade' to anything I have from ToolCraft?
ViniVidivici
12-26-20, 16:16
I had a similar issue with a PSA 8” .300bo upper and a PSA premium phosphate BCG.
Cycling by hand I could feel the BCG carrier had more resistance in the upper than any other AR I own.
It would cycle supers but not subs unsuppressed.
Swapped it with the Toolcraft nitride BCG in a PSA 16” .300bo 16” upper. Now both run unsuppressed subs just fine.
Just tolerance stacking.
I've seen a Bear Creek Arsenal BCG that was that way....stiff as hell moving inside the upper.
Turns out the "head" of the cam pin was out of spec, too big, and was binding inside the top of the upper.
Filed it down to spec, all was smooth as glass, that BCG now functions flawlessly.
You sometimes see wierd things with lower tier manufacturers.
Anything is possible, that’s why I added the tolerance stacking part.
Just tolerance stacking.
Tolerances stack.
A bit here and a bit there can make for a questionable/or even wrong result, move some pieces around and "There you go, everything works."
If the design is properly toleranced on the drawings (and the M16 is), and the parts made to the tolerances specified, then stacking of tolerance will have no effect on operation. The only time "tolerance stacking" causes problems is if the tolerance are not designed correctly, or the parts are not made correctly.
That's the whole point of interchangeability. A rifle made from parts all at one end of the tolerance band should work just as well as ones made at nominal, or a random mix.
If the design is properly toleranced on the drawings (and the M16 is), and the parts made to the tolerances specified, then stacking of tolerance will have no effect on operation. The only time "tolerance stacking" causes problems is if the tolerance are not designed correctly, or the parts are not made correctly.
That's the whole point of interchangeability. A rifle made from parts all at one end of the tolerance band should work just as well as ones made at nominal, or a random mix.
Thank you for posting this, I was about to post similar statements. At work (engine engineering) we spend a lot of effort to ensure that the tolerances are specified such that the chances of all "in spec" parts creating an issue are a statistical impossibility. We hold our suppliers to >1.33cPk and the idea that that you just have to hope the AR parts you choose all happen to work out is kind of absurd. If your parts are made to spec by a capable supplier, THEY WILL WORK.
"Tolerance stacking" as it pertains to AR parts isn't a thing. If you have a gun that doesn't work when it comes together (say, feed ramps in barrel extension overhang the ramps in the upper), it's not due to a stacking of tolerances-- it's due to a single part being off. The probability of having all "in spec" parts not work is vanishingly tiny such that the armorer for the biggest organization you know of still hasn't seen it.
And if that last part is not true, then something in the tolerancing scheme of the parts is not properly performed and the DVA (dimensional variation analysis) need to be repeated and corrected. Most commonly IME the problem is that the supplier is measuring the parts incorrectly or interpreting the drawing incorrectly.
Averageman
12-28-20, 13:55
If the design is properly toleranced on the drawings (and the M16 is), and the parts made to the tolerances specified, then stacking of tolerance will have no effect on operation. The only time "tolerance stacking" causes problems is if the tolerance are not designed correctly, or the parts are not made correctly.
That's the whole point of interchangeability. A rifle made from parts all at one end of the tolerance band should work just as well as ones made at nominal, or a random mix.
So, if my cam pin is within tolerances, but just a smidge on the edge of tolerances and it goes in to my bolt, which again, also is within tolerances, but it passes all QC tests but again is just a hair off, then I put in a firing pin that,....You see where I am going, right?
So, if my cam pin is within tolerances, but just a smidge on the edge of tolerances and it goes in to my bolt, which again, also is within tolerances, but it passes all QC tests but again is just a hair off, then I put in a firing pin that,....You see where I am going, right?
No, honestly, I do not.
If your cam pin diameter is 0.3100" to 0.3105", and the hole in the bolt is 0.3120" to 0.3135", they will work correctly. You will have a 0.0015 to 0.0035" clearance.
If the hole in your cam pin is 0.164 to 0.167", and your firing pin diameter is 0.153" to 0.154", they will work correctly. You will have a 0.006" to 0.010" clearance.
I am not going to go through the three thousand or so interfaces in an AR just to show you that all the tolerances work out properly, but rest assured, someone, or a group of someones, has, and they do.
If you are a smidge off the tolerances, you are not within the design requirements, and all bets are off.
So, if you are buying parts that "passes all QC tests but again is just a hair off", you need to reassess who you are buying parts from. The whole purpose of Quality Control (QC) inspections are to ensure the parts are made to within the specified tolerance zone. If they "pass QC" but are a "smidge off", somebody is failing at their job.
So, if my cam pin is within tolerances, but just a smidge on the edge of tolerances and it goes in to my bolt, which again, also is within tolerances, but it passes all QC tests but again is just a hair off, then I put in a firing pin that,....You see where I am going, right?
QC is not just to ensure everything is “in spec” but that the average of the parts is centered within the spec. And it is assessing the variation within spec to make statistical calculations about the probability of something falling outside.
So you see, tolerances just don’t “stack” all at the maximum amount and all in the same direction. A design engineer sets the tolerances based on how capable a supplier as as well as what is required to meet design intent. For example, if you have a bolt pattern, you have to account not just for the variation in the true position of each threaded hole, but the size and true position of each hole in the through flange (what you are bolting to the hole.) Depending in the number of steps in the stack up, the analysis of the tolerances can become very sophisticated, quickly exceeding what you can do in excel. You need more statistical power from something like CETOL.
Read the Wikipedia article on six sigma and see if it makes sense why you don’t have all the tolerances stack up in the same direction and at maximum value.
No, honestly, I do not.
If your cam pin diameter is 0.3100" to 0.3105", and the hole in the bolt is 0.3120" to 0.3135", they will work correctly. You will have a 0.0015 to 0.0035" clearance.
If the hole in your cam pin is 0.164 to 0.167", and your firing pin diameter is 0.153" to 0.154", they will work correctly. You will have a 0.006" to 0.010" clearance.
I am not going to go through the three thousand or so interfaces in an AR just to show you that all the tolerances work out properly, but rest assured, someone, or a group of someones, has, and they do.
If you are a smidge off the tolerances, you are not within the design requirements, and all bets are off.
So, if you are buying parts that "passes all QC tests but again is just a hair off", you need to reassess who you are buying parts from. The whole purpose of Quality Control (QC) inspections are to ensure the parts are made to within the specified tolerance zone. If they "pass QC" but are a "smidge off", somebody is failing at their job.
Lysander is spot on.
Lysander is spot on.
Yes, and saved me from going off on a rant.
Dr. Bullseye
12-29-20, 13:22
We are finally getting to my pet gripe and a recurring theme on this board: "Why won't all my Gucci parts work together?" I am really enjoying this.
Lysander and Hohn have nailed the engineering aspect.
In this case of a BCG causing short cycling, the main factors at play are gas circuit leakage vs gas port size.
There are six critical interfaces between twelve part features in the AR gas circuit.
Excess gas leakage comes from one or more of these part features being out of spec and failing to create a "good enough" gas seal.
A gas port can be enlarged over "spec" to compensate for excess gas leakage and that particular combo can actually work.
However, in reality, this is making a second part out of spec to compensate for the first part being out of spec and breaks part interchangeability and can cause future diagnostic issues.
Essentially, this is a "hack" and NOT the correct way to assemble a weapon you can trust.
On the cheap end of the industry, there is a bit of an unholy trifecta that tends to feed on itself involving under powered ammo, leaky gas blocks/gas tubes/bolt carrier groups and over sized gas ports.
Companies interested in doing it right, expect all critical parts to be in spec and the system as a whole to perform at a high level.
We are finally getting to my pet gripe and a recurring theme on this board: "Why won't all my Gucci parts work together?" I am really enjoying this.
Because some don’t understand the engineering, some don’t know the tolerances of the part it connects to, and some are manufactured to a “better fit" rather than held to correct tolerance.
I know many do not care for oversize gas ports. I have oversize gas ports on 3 carbines and they run flawlessly on any and all ammunition including steel case Wolf ammunition. I've replaced bolts, bolt carrier groups, gas tubes, etc. purchased from several vendors and I have had zero issues.
Before the ammunition supply started to dry up the carbines were run hard weekly, they could have been maintained better and they continued to run.
Dr. Bullseye
12-30-20, 13:03
Charles and Boyle calculated "gas laws" based on pressure, temperature, tube size, tube length and so on. There are lots of other gas laws engineers use to calculate the correct size for gas blocks. To add to all the other variables, now we have guys wanting seven inch barrels, Engineers do this for popular lengths of ARs. They have done the thinking and calculating. Re-engineering a machine is going to cause problems or may cause problems everywhere. Successful seat-of-the-pants engineering usually happens when the product is poorly engineered to start with. Like a Chevy for instance. People hot rod Chevys because they were crap to start with. Try that with a Mercedes and there will be tons of problems.
Gas Laws
https://en.wikipedia.org/wiki/Gas_laws
Every time one of these engineering companies comes along with non-mil spec parts which they claim are better than mil spec and you start stacking these in an AR, how can there not be problems??????? Explain it to me again.
On how much leakage is tolerable:
The graph below is an experiment in which the leakage area increased and the pressure in the carrier cavity (cylinder) was measured. Line (1) is confusingly listed as zero (0) leakage, when it is actually the system in nominal condition, which for reference, is an equivalent leakage area of about 0.700 mm^2, that includes the barrel-front sight, front sight-gas tube, gas tube-carrier key, carrier key-carrier, and carrier-bolt interfaces. The other lines, (2), (3) and (4) are the cavity pressure with the listed leakage area. The leakage area was controlled by increasing the clearance between the gas tube and carrier key. Under normal conditions, the gas tube-carrier key interface accounts for about 80% of the total leakage.
https://i.imgur.com/BFYMqvX.png?1
(2) is about 2X normal leakage
(3) 3X
(4) 5X
The resultant bolt/bolt carrier velocities are:
(1) 63.32 fps (100%)
(2) 49.97 fps (79%)
(3) 44.29 fps (70%)
(4) 37.73 fps (60%)
This graph is based on a standard M16A1 rifle length gas system, for a carbine length the cavity pressures will be about 10% higher.
On how much leakage is tolerable:
The graph below is an experiment in which the leakage area increased and the pressure in the carrier cavity (cylinder) was measured. Line (1) is confusingly listed as zero (0) leakage, when it is actually the system in nominal condition, which for reference, is an equivalent leakage area of about 0.700 mm^2, that includes the barrel-front sight, front sight-gas tube, gas tube-carrier key, carrier key-carrier, and carrier-bolt interfaces. The other lines, (2), (3) and (4) are the cavity pressure with the listed leakage area. The leakage area was controlled by increasing the clearance between the gas tube and carrier key. Under normal conditions, the gas tube-carrier key interface accounts for about 80% of the total leakage.
https://i.imgur.com/BFYMqvX.png?1
(2) is about 2X normal leakage
(3) 3X
(4) 5X
The resultant bolt/bolt carrier velocities are:
(1) 63.32 fps (100%)
(2) 49.97 fps (79%)
(3) 44.29 fps (70%)
(4) 37.73 fps (60%)
This graph is based on a standard M16A1 rifle length gas system, for a carbine length the cavity pressures will be about 10% higher.
Wow! That is an excellent start point to diagnose gas issues. Thanks for this!
On how much leakage is tolerable:
The graph below is an experiment in which the leakage area increased and the pressure in the carrier cavity (cylinder) was measured. Line (1) is confusingly listed as zero (0) leakage, when it is actually the system in nominal condition, which for reference, is an equivalent leakage area of about 0.700 mm^2, that includes the barrel-front sight, front sight-gas tube, gas tube-carrier key, carrier key-carrier, and carrier-bolt interfaces. The other lines, (2), (3) and (4) are the cavity pressure with the listed leakage area. The leakage area was controlled by increasing the clearance between the gas tube and carrier key. Under normal conditions, the gas tube-carrier key interface accounts for about 80% of the total leakage.
https://i.imgur.com/BFYMqvX.png?1
(2) is about 2X normal leakage
(3) 3X
(4) 5X
The resultant bolt/bolt carrier velocities are:
(1) 63.32 fps (100%)
(2) 49.97 fps (79%)
(3) 44.29 fps (70%)
(4) 37.73 fps (60%)
This graph is based on a standard M16A1 rifle length gas system, for a carbine length the cavity pressures will be about 10% higher.
How is the pressure expressed in this graphic?
Steve Shannon
01-03-21, 03:07
How is the pressure expressed in this graphic?
Based on the label on the vertical axis, Mega Newton per square meter, or mega Pascal.
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For anyone who needs an upgrade, we have a few of these in stock now.
https://d2j6dbq0eux0bg.cloudfront.net/images/812059/1643559957.jpg
Can you be more transparent with who manufactures your BCG?
Its well known that BCM, SOLGW, etc use Microbest so I don't think its a NDA matter.
How is the pressure expressed in this graphic?
For those who prefer to think in American Standard Units:
https://i.imgur.com/lR0TyTO.png
The resultant bolt/bolt carrier velocities are:
(1) 63.32 fps (100%)....
Not crucial to the discussion here, but I think the cited carrier velocities are much too high — do they agree with other sources you might have? I found an old report on DTIC that claims something like 17 fps. Also, a speed of 63 fps for a mass of 1 pound (BCG + buffer) would imply a kinetic energy of 60+ ft-lb, far more than the 10 ft-lb of work you could get from a piston supplying 400 lb over a stroke of 0.3 in. Unit conversion error?
I may have read to wrong column in the data, or converted wrong.
1) 9.2 m/s (100%)
2) 7.9 m/s (85%)
3) 7.3 m/s (79%)
4) 6.3 m/s (69%)
At least I wasn't shooting a spaceship to Mars or anything like that.
We wouldn't source from them if they weren't the BEST. :cool:
Can you be more transparent with who manufactures your BCG?
Its well known that BCM, SOLGW, etc use Microbest so I don't think its a NDA matter.
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