Some notes on HPT/MPI

AR/M4 AR Technical Discussion
1

Mil-C-70599A is the MILITARY SPECIFICATION for the CARBINE, 5.56MM: M4. This document is readily available online dated 20 January 1994. It is exactly what the name implies. Relevant sections for this discussion are:

3.4.4 High Pressure resistance. Each barrel assembly and
bolt shall withstand the firing of one Government standard M197,
5.56mm high pressure test cartridge conforming to MIL-C-46936.
After proof firing, parts shall be free of cracks, seams and
other injurious defects as evidenced by visual and magnetic
particle inspection.

4.7.4. Barrel inspection. The barrel assembly shall be
magnetic particle inspected in accordance with MIL-STD-1949
utilizing a current of 400 to 500 amperes for circular continuous
magnetization. The barrel assembly shall be examined for
evidence of cracks, seams and other injurious defects.

4.7.4.3 Bolt inspection. The bolt shall be magnetic
particle inspected in accordance with MIL-STD-1949 utilizing
standard five turn magnetizing coil with a current of 200 to 300
amperes. Both circular and longitudinal continuous magnetization
with wet fluorescent solution shall be used. The bolts shall be
examined for evidence of cracks, seams and other injurious
defects.

4.7.4 High-pressure resistance test. This test shall be
performed during individual carbine testing in accordance with
TABLE III, using a fixture for holding the bolt and barrel
assembly per drawing 11837944.
36
MIL-C-70599A(AR)
4.7.4.1 Test cartridge. One (1) high-pressure test
cartridge (see 3.4.4) shall be fired in each bolt and barrel
assembly. Unless otherwise specified, the bolt and barrel
assembly shall be tested concurrently. After proof firing
cartridge cases shall be examined for bulges, splits, rings and
other defects caused by defective chambers of the barrel
assembly.

The document makes reference to the following other military documents relative to the above

MIL-C-46936 Cartridge, 5.56mm, Test, High Pressure, M197
MIL-STD-1949 Inspection, Magnetic Particle (this is a 27 page document, also available online)
INSPECTION EQUIPMENT DRAWINGS 11837944 - Barrel and Bolt Proof Testing Fixture

There seems to be a lot of discussion of the minutia of the above in various locations recently. In order for one to have an intelligent discussion on the topics, knowledge and understanding of the above documents is necessary. Additionally, it is very easy for a manufacturer to say “we HPT” and/or “we MPI”, when in fact what we as consumers should be looking for is “we HPT in accordance with MIL-C-46936” and “we MPI in accordance with MIL-STD-1949”.

This by no means excludes other means or methods of quality control. However, just like a builder must demonstrate how their way is better than the building code if it does not meet it, manufacturers should be expected to demonstrate how their method of quality assurance exceeds the requirements of the relevant documents above.

2

Note, the initial document is dated 20 Jan 1994. I also have a copy dated 31 Jan of the same year. It is entirely possible (and even likely) that there are later documents, but since the reality is that most manufacturers aren’t even meeting the 16-year-old spec it is doubtful that they are meeting a newer spec.

3

Thanks for the info Rob!

4

TM43-0001-27 has info on the M197 on page 10-9.

5

Thank you rob s. I have read through lots of your articles and I appreciate everything that you do. Seriously, thanks.

OK, so unless an manufactuere/vendor specifically states that their barrel or bolt has been “HPT in accordance with MIL-C-46936” and/or “we MPI in accordance with MIL-STD-1949,” we can probably assume it wasn’t. Correct?

Bolt-related questions (I apologize in advance if these are stupid questions):

A reputable manufactuer of high quality non-military AR-15 match barrels offers a ‘mil-spec’ matched/headspaced bolt as an option with the purchase of a barrel. Without the inclusion of the applicable MIL-STD in their verbal or printed description, is it assumed that the bolt was not subjected to or does not meet the individual HTP or MPI standards? I’ll guess yes. So is this an issue when someone goes to build an SPR or DSM-R?

I have read that individually HPT and MPI test bolts and carriers should be marked as such, yet there is no mention of this in any of the military standards. Is this BS? I recently obtained a Daniel Defense auto BCG, and unless my eyes completely failed me, there were no markings on the carrier or the bolt indicating it was subjected to HPT or MPT testing, despite the manufacturer literature stating so.

Thanks again rob s.

6

First, I do not believe that there is any reason to HPT/MPI a carrier.

Second, Mil-C-70599A DOES call for marking of bolts and barrels that have passed the tests

4.7.4.4 Proof Markings. Proof marks and magnetic particle
inspection marks shall be applied on barrel assemblies and bolts that have passed this test.

7

Let me also reiterate that the point of this thread is to act as a jumping off point for discussion. I am NOT saying that any bolt or barrel that hasn’t been HPT/MPI is immediately crap. What I am trying to do is make sure that those tossing those terms around are clear with what they mean according to the military specifications as a starting point for the conversation. If, for example, someone can demonstrate that cryogenically treating a bolt and then x-raying for flaws is a better process then that should be considered.

8

I’m glad you posted that, as this is where I was headed next.

M193 Chamber pressure…52,000 psi
M196 Chamber pressure …52,000 psi (tracer)
M197 Chamber pressure…70,000 psi (high pressure test)
M855 Chamber pressure …55,000 psi
M856 Chamber pressure …55,000 psi (tracer)
M995 Chamber pressure…50,250 psi (armor piercing)

Therefore the High Pressure Test M197 round is 33% higher pressure than (or 135% of) M913, and 27% higher than (or 127% of) M855.

9

Sorry. I must have misremembered what I read. That is also stated word for word in MIL-R-63997B (AR) for the M16A2.

Just because something is stated as being ‘mil-spec,’ doesn’t necessarily mean it adheres to the exact military specification for that item. And even though a manufacturer states it’s product has been HPT and/or MPT, it doesn’t necessarily mean that those proceedures were carried out in accordance with a military standard.

I understand what the military specifications are and why they are in place. I also feel the term ‘mil-spec’ is used quite loosely in the consumer market. ‘Mil-spec’ in that the bolt has been properly manufacturer, tested, but mysteriously not marked? Or ‘mil-spec’ in that it was made to a standard dimension and will work with ‘any mil-spec’ carrier?

10

I believe the test is the most important. Markings are meaningless as they can be applied when the test has not been performed. Integrity comes into play both ways.

The point of this thread is sort of to point out that “all HPT/MPI is not created equal”. :wink:

11

It really comes down to trust and reputation of the manufacture. Anyone can say (and many do) that their product are milspec but how can one verify there claim? The military has the luxury of having someone looking over the shoulder of the manufacture making sure that that they are in compliance.

12

I distrust any blanket “milspec” claim. If you look at the way BCM lists out their products they say exactly what specs it is they are conforming to, not just simply saying “milspec”.

We addressed this in the Chart early on when the discussion turned to the term and it was generally agreed that it would be best to take the specs one by one and remove any mention of “milspec” or “TDP” from the Chart. When I wrote the Explanation of Features I tried to keep that same idea in mind and explain why a particular item, from the spec, was desirable.

Obviously it still does come down to the trust issue. With consumers becoming more educated terms like “HPT” and “MPI” have replaced the blanket “milspec”, but many people throwing around the terms don’t now what HPT really is any more than they knew what milspec really was.

Which is part of the intention of this thread, to get out there the information and where to find it for those that care to know the gritty details.

13

MIL-STD-1949A lists several ASTM standards as references and includes them in the standard. Some interesting things when you start burrowing into that…

ASTM E1444-01 “replaces MIL-STD-1949” but is superseded by ASTM E1444-05. ASTM E1444-05 references ASTM A275 “Standard Practice for Magnetic Particle Examination of Steel Forgings”, which also appears in MIL-STD-1949.

Confused yet? I’m getting there! :smiley:

14

Can anyone give a reasonable estimate regarding what percentage of barrels don’t pass HPT?
Or, what % can be expected to fail, more or less?

15

This is speculation

I would guess that the HPT is looking for either bad raw materials or bad heat treat. The percentage of failures would probably be very low. To me this would mean below .5% If a bad batch of barstock or improper heat treat makes it that far, I would expect high failure rates, as in over 10%.

Any manufacturer that is going through the trouble of doing those tests (Colt, BCM, Noveske…) is buying premium raw materials, and the instances of substandard bars should be rare. I can easily envision entire batches of barrels going through test with zero failures.

I’m a metallurgical engineer with some barstock experience in shafts for high-horsepower transmissions (up to 3000 hp), and in government specifications. That experience with high quality barstock is what I based my SWAG on.

Maybe we can get one of the manufacturers to chime in with real data.

16

Bud, just to be clear the part does not fail HPT. HPT is used to stress the part, and then MPI is the pass/fail.

I know of no manufacturers that are willing to release the percentage of failure information publicly.

17

Correct. Everyone from BM to Oly uses the term “Mil-Spec.” When they use it, they mean that all parts will FIT together. So you can take their upper and stick it on an M4. So in their little minds, they ARE Mil-Spec. :rolleyes:

The next question that should be asked when companies say that they HPT/MP is, what are your exit criteria from the MP test? What is acceptable and what isn’t?

C4

18

From conversations I have had with various companies, I would say that they failure rate is around 3%.

C4

19

Yes. I should have been more clear.

That’s what I get for trying to respond in a hurry.

20

BCM seems to be straight forward about what they do.

HPT / MP Inspected Barrels

Military specifications require that barrels undergo a HPT (High-Pressure Test) load as a part of quality assurance procedures. Commonly referred to as a proof load, the M197 HPT is rated for 70,000 psi. BCM barrels are proof loaded and then MPI (Magnetic Particle Inspected) to detect any possible flaws in the barrels surface. The barrels are MP inspected with both circular and longitudinal magnetic fields per ASTME1444-01 (current Mil-Spec) to assure a high quality finished product. All BCM barrels are marked HP MP as our proof load marking.

The term milspec is meaningless. Go to any gun show and everything you see will be milspec. It’s become to broadly used to sell stuff from people that don’t understand what it means to people who don’t understand what it means.

I also went to the Daniel Defense sight and notice that they make no claim to HP test, just MPI.