Dependable BCG. Which one?

[georgeib]

Just a heads-up.

I've had not one, but TWO JP enterprises FMOS carriers fail-
Last weekend was number two. The front gas key screw broke the head off, of course the carbine will stop cycling.

I sent the first one back to them for repair, then got rid of it later as it still wouldn't cycle.

This one, I just repaired it myself with new screws. I figure at least I'll know it's done right, and not a refurbished swap, which I think the other one was.

Sad, as the body is beautifully machined.

Have you checked to make sure that the carrier key isn't contacting the receiver at full compression? Just drop the buffer into the buffer tube without the spring, and then insert the back of the BCG. Make sure there's at least 1/16" clearance between the gas key and the receiver.

[AndyLate]

Have you ever used a Rguns BCG????? I know 13 that are being used and no reports of problems. Two have around 5k rounds, one is around 3K. One had a set of gas rings go bad, but I don't remember if it was on a Rguns bolt or someone else's bolt. (OH, Thanks for making me remember, that guy has not returned a new set of gas rings to me, I will have to call him, I may have to foreclose on that unit, OH YES! lol) He replaced MY set of new gas rings and was up and running and I have not heard of any other problems from him.

You say "there is zero evidence Rguns bolt carrier groups belong in the same list as (example only, not all-inclusive):Bravo Company, Colt, Daniel Defense, FN Herstal" Do you have any evidence that Rguns BCG's are not dependable? That is the question that I was asking the guy's on M4Carb. Has anyone used Rguns BCG's and had any problems with them? I have not seen any problems with Rguns BCG's so far.

It honestly would not matter if I have seen or not seen a bad Rguns BCG, I have been more than clear I am not an SME. What I can say with confidence is the BCGs that people who are SMEs recommend are dependable. I do not recall an SME on this site recommending Rguns BCGs.

Andy

[Hammer_Man]

I love it when marketing types try to make their ads sound "technical" . . .

"Mil Spec Extractor
4140 Steel per ASTM A108, cold drawn and stress relieved BHN 187-229

Mil Spec Gas Key
4130 Profile Extruded Steel
Grade 8 Hardware"

I hope their extractor is not Brinell 187-229. That is soft, annealed soft, about a 9 on the Rockwell C scale.

After heat treatment the extractor should be Rockwell C42 to C44

And the infamous, and misunderstood "Grade 8" bolt . . .

First, the only fastener specification that has Grade 8 in it is SAE J429 which deals with hex head bolts and studs, and the "Grade 8" bolts and stud start at 1/4 nominal thread diameter. I am pretty sure you don't have a 1/4 bolt holding your gas key on.

Second, if we just take the material strength requirements for "Grade 8" bolts, that means they are made from a steel with a minimum tensile strength of 150,000 psi. The actual screw per the Army's drawings is a item peculiar screw made from a nonstandard (screw) steel and with a nonstandard head, but we'll skip that fact for the moment and look at the standard "mil-spec" screws that are the closest the the 8448508 (19200) drawing. These screws would be the Aerospace Industries Association National Aerospace Standard NAS1351 - Screw, Cap, Socket Head, Undrilled and Drilled . . . yada, yada".

NAS1351 screws are required to be made from steel with a minimum yield strength of 180,000 psi.

In fact, all of your MS and MIL-S socket head cap screws are required to be made from steels with a minimum yield strength of 170,000 to 180,000 psi, 10% to 20% stronger than your "Grade 8".

I have no idea what the required heat treatment specs are for the extractor, or the gas key. I did a quick Google search for those specs, and couldn’t find anything. I’ve used two of these before without issues, and just ordered my third. If this one blows up because the extractor isn’t hard enough I’ll let you know.

[Core781]

Cool info. Might explain why grade 8 screws are shearing off.

[AndyLate]

Cool info. Might explain why grade 8 screws are shearing off.

I would say it explains why grade 8 screws are NOT shearing off or breaking when used in BCGs. Whoopie Goldberg' MAGA hat doesn't shrink in the wash, either.

Andy

[Uncas47]

Always grateful for lysanders knowledge bombs, I find myself combing threads for anything he chooses to add.

[Core781]

I would say it explains why grade 8 screws are NOT shearing off or breaking when used in BCGs. Whoopie Goldberg' MAGA hat doesn't shrink in the wash, either.

Andy

I have had some military tech ones let go: under abnormal circumstances. I have also seen some dude on ebay selling "grade 8" screws as replacements. I was at a armorers course years ago and someone had some spares and I swear they must have been from the hardware store, when I compared them to a set I had from a well respected manufacturer they were not the same.

[AndyLate]

I have had some military tech ones let go: under abnormal circumstances. I have also seen some dude on ebay selling "grade 8" screws as replacements. I was at a armorers course years ago and someone had some spares and I swear they must have been from the hardware store, when I compared them to a set I had from a well respected manufacturer they were not the same.

There is a difference in quality between fasteners, no question. I was referring to Lysander clarifying that grade 8 screws are not made in the size used for gas keys/BCGs.

Andy

[lysander]

First off, I AM NOT saying that RTB makes bad stuff. It probably is as good as most everyone else's stuff, because most retail sellers get their stuff from the same few manufacturers, and even the cheap stuff is adequate.

What I was saying was most often the guy that writes the blurb for the catalog page has little understanding of the technical specifications, and grabs out-of-context snippets and common (if incorrect) jargon to fill out the "Technical Specification" tab, under the part.

As to carrier key cap screws in particular:

The screw specified on the Colt's drawing is Screw, Cap, Hex Socket Head, DWG No. 8448508. It is NOT a standard screw, so you won't find it at the hardware store.

Standard screws are ASTM A574-13 and NAS 1352, which allow for steels with the following composition:

Carbon - 0.31%
Phosphorus - 0.035%
Sulfur - 0.040% max
one or more of the following to ensure strength requirements are met - chromium, nickel, molybdenum, or vanadium (That covers a lot of steel alloys.)

The mechanical requirements are:

Tensile strength - 180 ksi
Proof load - 140 ksi
Hardness (Rc) - 39-45
Yield at 2% - 153 ksi
Elongation - 10% min (Most 41XX and 43XX series steel this is 15% to 20%)
Reduction in area - 35% min (Most 41XX and 43XX series steel this is 40% to 50%)

DWG 8448508 calls for AISI 4037 heat treated and tempered to 15N 78.5 to 83 (HRc 36 to 45). At this hardness 4037 will have the same ultimate and yield strength, but the elongation will be only 5% and the reduction in area of 36%

So, what the difference? It's all steel with the same yield strength, right?

Well, the strength and hardness are the same but the other properties have a bearing on the behavior of the screws and explain why Armalite/Colt's was so specific in material choice. The elongation at break is 5%, and 36% reduction in area for 4037, the other alloys have values in the 15% to 20% elongation and 40% to 50% reduction in area. This means that 4037 is a very "stiff" alloy, in takes a lot of force to stretch the material a small amount

Also, the required torque on the 8448508 screws is 58 in-lbs, which means the clamping force is between 1,793 and 2,080 lbs. Clamping force is the result of the steel of the bolt stretching like a rubber band and holding tension.

The proof load for 8-32 UNC ASTM A574 screws is 1,960 lbs. Proof load means the bolt stretch at this load remains inside the elastic region of the stress-strain curve. Loads above the proof load can result in permanent bolt stretch and loss of torque and clamping load. So, some ASTM A574 screws can stretch under the carrier key specified torque. 4037 screws, being stiffer, do not stretch as much and therefore retain their clamping load. This is why BCM, Colt's, and other quality places use the 4037 screw.

(You can mitigate the bolt elongation problem by simply reducing the torque to about 50 - 55 in-lbs and making sure the screws are installed dry. You still maintain a 1,500-1,700 lbs of clamp.)

Also, there is that one dimensional difference, ASTM A574 and NASM1352 screws specify a 9/64" (0.140") key and the DWG 8448508 screws specify a 1/8" (0.125") key. Since the head diameter is the same, the drawing screw has a thicker web. This gives a visual cue to the type of screw, and the thicker head web resists stripping (a little bit).

Does it matter? I don't know, that's up to you. Personally, I can live with the minor differences, but just be aware that there are differences.

[Core781]

There is a difference in quality between fasteners, no question. I was referring to Lysander clarifying that grade 8 screws are not made in the size used for gas keys/BCGs.

Andy

Makes me wonder what in the hell is floating around when folks source non milspec gas key screws...