Question on barrel MPI

[Grand58742]

Kind of an odd and off the wall question, but when is the magnetic particle inspection performed on a barrel? Is it prior to the machining while the barrel is still a blank or after the barrel has been turned and bored out? Maybe bored out is the wrong technical term, but after the bore and rifling gets machined into it?

And second part is it after the HPT for those manufacturers that do so? I mean, it would make more sense to do so after a high pressure test, but I'm curious.

Reason the question came up was the answer from a manufacturer that just didn't sound right on TOS. (Yeah, I know, it's TOS) Person asked what the differences was in a Colt and another brand and was given the answer. I made the comment the manufacturer in question did batch testing on their barrels and bolts. Got called on it (even though the manf. in question stated they batch tested their bolts) but nothing out there on the barrels. So I asked the manufacturer directly and here's the answer I got:

http://www.ar15.com/forums/t_2_489/220104_Barrel_testing_.html

I know the HPT part is pretty much BS, but wasn't sure whether a manufacturer could claim their barrels were "magnetic particle inspected" in good faith since the MPI was done prior to the final work being performed. Wouldn't machining potentially cause additional stresses on the steel that would show up on an MPI after completion?

Not that I plan on buying the brand in question, but just for my own knowledge and be able to be armed with the information in the future and call BS when needed. The answer just didn't sound right to me.

I tried the search function, but unfortunately the question is too specific to get an answer.

[glockeyed]

High Pressure Test (HPT) Bolt

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This is also sometimes referred to as "proof loading" or "proof firing". Essentially it is a test fire of the bolt (and barrel) in order to subject the part to a pressure that could cause it to fail in use. This is a preparatory step in order to prepare the part for the next step. Not all companies perform this step and prefer to "batch test" their bolts and barrels, or to test a representative sample of each batch.

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More information on proof testing can be found here

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http://en.wikipedia.org/wiki/Proof_test

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Magnetic Particle Inspection (MPI) Bolt

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Like shot peening, there are resources available online as to what the exact process is and how it works, but the intended purpose is to check for surface cracks in the part that may not be detectable by the naked eye and that may have been caused by the HPT. Not all companies perform this step and prefer to "batch test" their bolts and barrels, or to test a representative sample of each batch. This is a crucial step following the HPT in order to observe the results.

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More information on MPI can be found here

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http://en.wikipedia.org/wiki/Magnetic_particle_inspection

quoted from rob_s's The Chart™ :p
https://spreadsheets.google.com/pub?key=pwswheghNQsEuEhjFwPrgTA&single=true&gid=5&output=html

[Grand58742]

Kind of partially answered it. I checked the "Chart" and the Wiki articles prior to posting. I see in the notes the HPT is done prior to the MPI and the MPI is used to gauge the results. At least from the barrel side of the house.

But the question is could additional machining on barrel blanks cause problems if the material had passed an MPI in the first place? I.E. nothing that showed initially, but the stress from the reaming causing problems on the finished product.

It almost seems from the link they are putting the cart before the horse. Doing an MPI prior to producing a final product.

[glockeyed]

well, since HPT is firing a "hot" round out of the barrel, i'd assume most of the machining is done.

[Grand58742]

well, since HPT is firing a "hot" round out of the barrel, i'd assume most of the machining is done.

Here is what was said specifically...


The barrel steel is MPI tested before the barrels are turned.

At the link I first posted. To me "before the barrels are turned" implies the barrels had undergone the MPI prior to being reamed.

Or am I not understanding what "turned" is meaning.

Hence my "cart before the horse" comment.

[GTifosi]

'Turned' would be the outer profile being cut.

A barrel blank is bored and rifled before the outer profile is turned as this gives more physical support of the material for boring.
ie: the inside of a thick walled tube is easier to work on than a thin walled tube if you don't want warp, wobble and tear through.

However, this doesn't mean the OD is massive and it all gets hogged down to final profile. It may only be .020-.050-100 thousandths oversize at the chamber.
This amounts to a few pretty low pressure skim cuts which won't effect what HPT and MPI actually test (expansion cracking from internal force)

The process would most commonly be get blanks roughed out to X OD, bore, rifle, HTP/MPI, turn profile, true the tube, then do the inner and outer coating, drill gas port and affix FSB/gas block if part of the order.

Some variation between steps from manufacturer to manufacturer, but the HPT/MPI would fall in where it does above as it makes no sense to fully profile a barrel before finiding out if the blank is junk.

[Grand58742]

Thank you