Overlooked BCG components

[calvin118]

There are a lot of companies advertising a BCG with a shot peened/MPI Carpenter 158 bolt and a 8620 carrier with a 4140 gas key and grade 8 screws, properly staked. That is what most companies seem to focus on, and what most people bring up in the "just as good" conversations. My question is, are there other critical components?

I was looking over Toolcraft bolt carrier groups (which are respected by some members here) and noticed that the cam pins are listed as 8620 steel. It is my understanding that the spec calls for 4340. I know that 8620 bolts used to be a serious problem when they were more prevalent. Cam pins are certainly a high stress item that can break. Is the 8620 cam pin a liability?

Additionally, the extractor is advertised as being made from D2 tool steel. I do not believe this is what the spec calls for either.

What are the ejectors made of?

I know the general advice is to get a Colt or BCM bolt carrier group for the "known quality". I am not planning to put a Toolcraft or PSA BCG into a firearm that anyone is going to have to rely on for anything serious. Rather, I just wanted to raise the issue of whether there are important details beyond those that are commonly cited in these conversations, and what they might be.

[Iraqgunz]

Bolts were never made from 8620 to my knowledge.

[AndyLate]

My post did not address the OP intent..

Andy

[Sry0fcr]

I just wanted to raise the issue of whether there are important details beyond those that are commonly cited in these conversations, and what they might be.

Probably, but this is an engineering question so I wouldn't seek advice from anyone who isn't one when it comes to components that are critical to the function of the operating system.

[alx01]

You might want to save this page as a reference: https://sonsoflibertygunworks.com/co...ant=3084070593

SOLGW is not a large company, but seems to be doing right things in terms of quality and openness with the customers about their specs. [I'm not affiliated with them in any way].

Most companies are focusing on materials and "specs" whatever it means. I think that materials are definitely important, but in my experience correct tolerances, manufacturing processes, and proper treatment of materials are MORE important than the base materials when it comes to durability and longevity of the product.

Sons of Liberty Gun Works Bolt Carrier Group HPT/MPI

Bolt:
Precision machined from Carpenter Technology No. 158 alloy,
Heat treated per mil-spec,
Shot peened per ASTM B851/SAE AMS2430S,
Vibratory tumbled
Individual high pressure/proof (HP)
Full Auto test fired as an assembly
Individual magnetic particle inspection ASTM E1444/E1444M (MP)
Markings deep laser engraved (SP=Shot Peened, HP=High Pressure Tested, MP=Magnetic Particle Inspection.)

Extractor:
Precision machined from 4340 tool steel alloy per SAE AMS6415T/AMS6484D (vs 4140),
Heat treated
Shot peen per ASTM B851/SAE AMS2430S.

Extractor Retaining Pin:
Precision machined and ground S7 Tool Steel
Heat treated per ASTM A681-08
Shot peened per mil-spec print
Manganese phosphate

Extractor Spring:
Tactical Springs/Sprinco 5 coil, ASTM Grade A401 Chrome Silicon wire stock
Heat treated
Stress relieved, Molybdenum-Disulfide infused
Cryogenic processed (all post winding)
Mil-spec black insert/synthetic elastomer extractor buffer (nitrile-butadiene, shore hardness 80+/-5 durometer per MIL-PRF-6855)

Ejector:
S7 Tool Steel
Heat treated per ASTM A681-08
Shot peened per Mil-Spec
Manganese phosphate

Ejector Spring:
Heat treated
Mil Spec

Gas Rings:
Standard stainless steel per Mil-Spec X3

Carrier:
Machined from AISI 8620 aircraft qualty alloy per ASTM A108/A322-13
Full auto profile
Heat treated
Carburized/strain relieved, hard chrome bore
Precision ground gas key interface
Sons of Liberty Battle Flag logo deep laser engraved
Manganese phosphate

Gas Key:
Machined from 4130 chromoly steel alloy per ASTM A108/A322-13
Heat treated, hard chrome internal
Manganese phosphate
Permatex sealed
Grade 8 hex screws torqued and staked all per mil-spec

Cam Pin:
Precision machined 4340 chromoly steel alloy per SAE AMS6415T/AMS6484D
Heat treated/processed per mil-spec
Manganese phosphate
Solid film lubricant applied and cured per mil-spec

Firing Pin:
Precision machined 8640 steel alloy
Heat treated/processed per mil-spec
Hard chrome plate

Firing Pin Retaining Pin:
Precision machined from 1038 carbon steel
Heat treated/processed per mil-spec
Manganese phosphate

[lysander]

There are a lot of companies advertising a BCG with a shot peened/MPI Carpenter 158 bolt and a 8620 carrier with a 4140 gas key and grade 8 screws, properly staked. That is what most companies seem to focus on, and what most people bring up in the "just as good" conversations. My question is, are there other critical components?

I was looking over Toolcraft bolt carrier groups (which are respected by some members here) and noticed that the cam pins are listed as 8620 steel. It is my understanding that the spec calls for 4340. I know that 8620 bolts used to be a serious problem when they were more prevalent. Cam pins are certainly a high stress item that can break. Is the 8620 cam pin a liability?

Additionally, the extractor is advertised as being made from D2 tool steel. I do not believe this is what the spec calls for either.

What are the ejectors made of?

I know the general advice is to get a Colt or BCM bolt carrier group for the "known quality". I am not planning to put a Toolcraft or PSA BCG into a firearm that anyone is going to have to rely on for anything serious. Rather, I just wanted to raise the issue of whether there are important details beyond those that are commonly cited in these conversations, and what they might be.

The choice of steel can sometimes be somewhat subjective. In cases like this there are several alloys that are more than adequate for the jobs at hand. The difference between 8620 or 4340/4130 in the cam pin application is a bit of a wash, either are good for the job but each have their own strong points.

The cam pin is loaded almost entirely in shear, and both steels have more than enough strength to withstand the shear loading. 8620 is slightly harder than 4340 when carburized, so it will be slightly better at resisting rubbing wear, but has slightly lower toughness, and may have slightly less fatigue resistance. But then again, the cam pin is not really a fatigue sensitive part. It fact, when was the last time you saw a broken one (not one that has worn spots, but sheared through)?

The same goes for extractors, by the print they are also 4340, but the substitution of of that for D2 tool steel is a fairly straightforward one. D2 tool steel is a high abrasion resistant, extremely strong, low distortion, semi-stainless steel. It does not have the same toughness as 4340, but it is still quite high. Either D2 or 4340 will do quite well.

Ejectors are made from S2 shock resistant tool steel, by print. It has extremely high toughness, as the name suggests, is for use in applications where it is required to take a beating and not crack, peen, or deform. All of your tool steels have very high hardness.

As an aside, 8620 should be fine for a bolt, if the heat treatment is done right. However, the only time I have heard of 8620 being used was after a panic and AR parts were in high demand and low availability, and some "econo-supliers" used substitute materials to boost profits. It cases like that, strict adherence to proper heat treating is questionable.

Which brings me to a point worth making: "The quality of the manufacture: adherence to tolerances, proper heat treating, etc, is more important than whether the parts is made from the exact steel called out on Colt's drawing, or another equally good substitute material."

Buy from folks you trust.

[LRRPF52]

One of the several areas that I look at on all my BCGs is the carrier bore for the bolt and how it was machined.

If there are rough tool marks inside the carrier bore, even after hard chrome plating, those rough cuts will shred gas rings rather quickly and short-stroke a gun after only a few hundred rounds in some cases.

They either need to be polished or rejected. Even some of the most prestigious AR15 manufacturers have had carriers go out like this that needed to be finely sanded and polished.

The gas rings are another critical set of parts that get overlooked, and not all of them are made equally. If too brittle, they'll chip. If too soft, they'll gall and wear quickly.

Firing pins are another critical component that I've seen issues with.

When made too soft or incorrectly, I've seen some that have pitted or deformed on the tips, causing issues with primer piercing and cutting, which allows peak pressure gas to flow through the tiny perforation in the primer cup and plasma-cut the bolt face.

In this depressed pricing market, a lot of new entrants and even old hands are looking for places to cut corners to make margins and keep their heads above water.

It makes me suspect where those corners are being cut, and I don't get a good feeling when I see them being cut in critical stress-bearing components.

[26 Inf]

Last two posts are examples of why this site is a great resource. :thumbsup: :thumbsup:

[calvin118]

Indeed; thanks for the excellent responses.

[AR-Tenner]

I would add my experience to the proposition that proper manufacturing, tolerances, and heat treatment are the most important. Both 8620 and 4140 were used early-on for bolts in the ARs development. Heck, the original Colt 601s had 6061 aluminum receivers and receiver extensions, and yet they are going strong. ArmaLite's modern AR-10 bolts are made from 8620, and I have over 25,000 rounds on one of them with nothing more than finish wear and the preventative maintenance extractor and ejector spring changes every 10,000 rounds. The springs were fine, but I change springs BEFORE there is a problem. I also have a 1959-production AR-10 made in Holland, and from my research contacts - I am the author of the recent book on the AR-10 from Collector Grade Publications - I discovered that the exact rifle I own was fired around 40,000 times in its 13 years in Mozambique, most of that fire in full-auto, and never had any part replaced, even springs. The bolts on those Dutch-produced AR-10s were 4140 at the very best, and were hard-chromed on all surfaces (even the firing pin retainer is chromed), and not even the bolt lugs, bolt face, or cam pin on this Portuguese AR-10 have any areas where the chrome has worn through, let alone cracked.

Toolcraft, if it has not been said here before, is a tier-1 builder. They have NAICS codes and produce BCGs for the military, and for various top-end OEM AR-makers. Either their 9310 or C158 bolts are excellent, and the C158 is their mil-spec one. Their carriers are also totally mil-spec, and their key staking jobs are tied with Colt with the best I have ever seen.