C158 Bolts vs. MPI

[Doc Safari]

If you stick to known quality, C158, phosphate bolts I would not worry about the marking or testing.

And where do Palmetto State Armory bolts rate? (I'll take a guess: Hobby Grade).

[tehpwnag3]

There are some data points to be found, but all you really care about is the service life of YOUR bolt (and perhaps, in time, you'll be able to find that out yourself). After all, we have no idea how stringent PSA's (or their vendor's) controls are for acceptance. You could have a bolt that wouldn't pass at BCM but passed with flying colors at PSA. And that bolt might last 10k rounds or 500. Who really knows....

That's why buzzwords like "match grade", and what "mil-spec" has seemingly become to some sellers, are meaningless because the product is as only as good as the QC/QA of the company that produces them.

I have a PSA AR pistol that the bolt is marked C158 MPI.

A friend's is only marked C158.

I wonder what the service life, on average, is for one of the PSA bolts.

[Doc Safari]

There are some data points to be found, but all you really care about is the service life of YOUR bolt (and perhaps, in time, you'll be able to find that out yourself). After all, we have no idea how stringent PSA's (or their vendor's) controls are for acceptance. You could have a bolt that wouldn't pass at BCM but passed with flying colors at PSA. And that bolt might last 10k rounds or 500. Who really knows....

That's why buzzwords like "match grade", and what "mil-spec" has seemingly become to some sellers, are meaningless because the product is as only as good as the QC/QA of the company that produces them.

Agreed. I was looking for feedback on track record and historical data.

[26 Inf]

Just because something was MPI’d, doesn’t mean that it actually passed to an acceptable standard, it just means they looked (at least that’s my understanding).

I'm pretty sure I understand the basics of magnafluxing/mpi and I don't see how you can do anything but accept or reject on a bolt - a crack anywhere should be grounds for rejection.

[26 Inf]

Agreed. I was looking for feedback on track record and historical data.

Doc - I'm going to be a jerk and suggest that you are trolling for a long drawn out discussion.war. As often as you post, and have belonged to the forum, you surely must have known C-158 was the mil-spec, and you can't tell me that you don't already know where folks come down on anything branded PSA.

Just in case - Carpenter-158 (C-158) is a proprietary steel alloy manufactured by Carpenter Technology. So, PSA's vendor is not buying C-158 from the steelill with the sign out front that says 'The We Sell Shitty Steel to PSA Steel Mill,' only one place makes C-158.

The reason that folks might use 9310 is that it is a 'generic' alloy, not limited to one manufacturer, therefore, it is more available, and available in smaller quantities than C-158. I've also read that cost is lower. I do not know if that is a mechanism of economies of scale, or the actual cost to produce the alloy.

Here is a post by Bill Alexander, who is apparently knowledgeable about the two steels:

It is always interesting to consider the bolt within the context of its application. To do so will draw not only on stress analysis, but also on fatigue theory and metalurgy. This will quickly move the solution beyond the simple question of which steel is best, for the best steel if applied out of context will not perform adequately.

So for simple illustration let us assume that the steels are applied well, before discussing the differences.

Carpenter 158 is without doubt an excellent material for the production of M16 bolts. The material is clean with negliable elements in the make up that are detrimental to the fatigue life. It can operate happily within the confines of the enviromental requirements imposed by the application and has a very low deformation of the parts as they run through heat treatment. The down side to the material is that it was designed to heat treat in large sections so the thin bolt material will respond somewhat voilently. Again not an immediate problem if the heat treatment is absolutely perfect but within the confines of a production enviroment it will throw problems.

By comparison AISI 9310 will on first inspection also make an adequate bolt material. It has several alloy elements that promote a better structure and in the correct heat treatment will provide a slightly higher toughness than Carpenter 158 which is benificial to the fatigue life. Corrosion resistance is slightly higher but as with C158 it should not be applied without some form of surface protection. Thin section response to quenching is somewaht less than C158 which makes it better suited to the manufacture of bolts. However by comparison to Carpenter 158, AISI 9310 has several elements present in its composition that are detrimental to fatigue while not being evident in the physical properties.

It has become evident from the industry that a number of manufacturers have jumped upon the AISI 9310 wagon in order to claim better performance. While in theory an AISI 9310 bolt may perform better I would not typically select this material specification. There are a wide number of superior alloys available without resorting to the nickel based maraging alloys which are expensive, difficult to machine and extremely temperamental in behaviour. The steel industry has advanced since the specification of Carpenter 158 but the basic premises for the selection remain even if the menu has now expanded.

Bill Alexander

If you use the search button........

[Doc Safari]

Doc - I'm going to be a jerk and suggest that you are trolling for a long drawn out discussion.war. As often as you post, and have belonged to the forum, you surely must have known C-158 was the mil-spec, and you can't tell me that you don't already know where folks come down on anything branded PSA.

Really?

Really. I've always stuck to Colt and BCM because they are trusted brands. I picked up a PSA pistol with KAK brace on a whim and noticed the bolt markings. I never have gotten much into material specs and whatnot.

I realize PSA is probably a B-grade fireaarm. I just don't want it to be a Z-grade firearm.

[Todd.K]

Bolts of normal material and finish tend to fail within a few hundred rounds, or after many thousands.

[markm]

There's really no reason to gamble on a bolt that's not from BCM or Colt.

I've had the aftermarket crap break on me, and the lesson was learned.

[Wake27]

And where do Palmetto State Armory bolts rate? (I'll take a guess: Hobby Grade).

That's likely not as straightforward of an answer because they sell multiple grades, unlike the better brands. They have at least three different BCGs off the top of my head - a basic phosphate, basic nitride, and "premium." I think they have some NiB as well. In theory, at least the premium is held to a higher standard, but I wouldn't be surprised if they had several different grades.

[Wake27]

There's really no reason to gamble on a bolt that's not from BCM or Colt.

I've had the aftermarket crap break on me, and the lesson was learned.

I'm less and less convinced of that, assuming its only for a toy gun or tertiary spare. The PSA premium BCGs are often on sale for like $60-70. That's the cost of just a quality branded bolt. I can get two of those and still have a little leftover instead of a well-priced BCM BCG. I think it could be worth it, as long as its understood that you're taking a gamble and are prepared. But if I have great BCGs in several great ARs and have one or two full backups plus small parts, I don't feel bad gambling on one of the premium ones in my range toy.