Hi Folks

I Just finished my first build (kind of, still waiting to replace the mil-spec charging handle w/BCM extended Gun Fighter as soon as its delivered). During the build I researched some of the parts determining what parts/specs to get. I decided the PSA Premium Full Auto HPT/MPI BCG (with Logo! LOL) would be a good choice.  During my research I found that a vendor can/does advertise a bolt if not individually stamped as HPT/MPI thru batch testing, which IMHO is total BS. But to the heart of my question. A lot of vendors stamp and individually test their bolts with MPI without the mention of HPT. MPI of a bolt proves its not cracked or fractured when its brand new, never fired, fresh out of production. It seems to me this is kind of an incomplete test if not HPT first? If the bolt is subjected to firing a far higher pressure than spec round then tested for fractures its proven to have quality strength. If there's no HPT before the MPI its only proven to not be cracked brand new out of the box? Am I missing something here?

You're deduction sounds reasonable to me.

HPT is a mixed bag with dubious benefits.  It's a destructive test that dramatically shortens the service life of a bolt.  This is a non-issue for the military which specs a 3K round service life for the M4A1 bolts.  In practical use, the machine gun rental range we contract to beat on prospective parts confirms that the functional life of non-HPT bolts far exceeds those of HPT bolts. HPT isn't even that good of an indicator that a bolt was made correctly.  In Jan. 2013, a manufacturer released a batch of incorrectly treated bolts which were individually HPTed and MPIed.  The bolts didn't start failing until consumers ran them.  Every one of them would fail within the first magazine of use but HPT didn't catch this.

 

Since I've learned more about HPT and how it affects bolts and the questionable benefits, I've stopped specifying HPT as a requirement for house branded bolts.  It may help catch certain types of defects but not all.

I'm with MPX on this. I don't sweat that anymore. I care what steel the bolt was made from, but not how it was tested.  Also today there are better steels then C158, so that should be considered the baseline minimum, not the ideal.

Interesting and makes sense. I guess if you take a brand new bolt and slam it extremely hard if you don't brake it you've still stressed/weakened it. The build I was referring to is a 20" bull barreled 9.5lb bench gun not a duty rifle. It will be a long time before it sees thousands of rounds. I guess the moral of the story is long standing reputable companies have a reputation for a reason. If PX or BCM was selling stuff that brakes they wouldn't  be the bench marks that they are........ take they're word for it it's good and stay away from PangWhan Bolts Unlimited and ya should be GTG

In speaking with Ty on this stuff, I have to agree with him wholeheartedly. It's weird to me that you would stress anything that hard and expect it to perform for a long time without failure. Stress is cumulative in materiels.

 

C

In speaking with Ty on this stuff, I have to agree with him wholeheartedly. It's weird to me that you would stress anything that hard and expect it to perform for a long time without failure. Stress is cumulative in materiels.

 

C

 

What about when firearms are proof tested?

What about when firearms are proof tested?

Anything pushed beyond is reasonable limits is going to have a shorter useful life. Guns, cars, condoms. Doesn't matter. Everything has a finite limit. If you stress something that hard it just can't last as long. As Ty (MidwestPX) pointed out, there is research to back that up.

 

If you're removing something from a batch, abusing it and throwing it away, different story. The end user is, obviously, not affected. The end user simply benefits from seeing how long the materials can last when abused. They are not being sold a used and abused package of goods.

Chris - I'm in total agreement.  But does anybody actually know what the HPI test procedure is?

 

Is it a spot check of the batch that consists of the square root +1 as is standard QC procedure?

 

If it is the case you may never even get a bolt that is HPI tested, just a spot check of the batch was tested. Do they dispose of the stressed bolt?

 

Just being the devil's advocate.  Because I think a lot of this "Quality Mfg" stuff is BS marketing.  The only out of spec part I've seen was Stag and they are somewhat highly regarded.

 

If something is MPI I know its intact at point of mfg, and that's good enough for me.

  But does anybody actually know what the HPI test procedure is?

 

 

I'm sure SOMEONE knows because it is their job to it. However, I'll bet it is basically the same as a barrel test, ie: shoot a proof load through it, or in the case of the bolt in front of it. 

 

MPI is more about finding voids and uneven metal so assuming the metal is good to go, I'm not sure MPI will tell anyone anything about how the HPT went. My guess is that if the bolt doesn't break it considered to have past HPT. Generally speaking I would prefer to know that MPI happened, not caring that much about HPT

I'm sure SOMEONE knows because it is their job to it. However, I'll bet it is basically the same as a barrel test, ie: shoot a proof load through it, or in the case of the bolt in front of it. 

 

MPI is more about finding voids and uneven metal so assuming the metal is good to go, I'm not sure MPI will tell anyone anything about how the HPT went. My guess is that if the bolt doesn't break it considered to have past HPT. Generally speaking I would prefer to know that MPI happened, not caring that much about HPT

 

That's how I feel about it.  That and the material.

Many people want HPT and MPI because the military demands it.  It's part of the "Well, if the military has it, it must be good" mentality.  If that were the case, I'd stock Grip-pods...  

Chris - I'm in total agreement.  But does anybody actually know what the HPI test procedure is?

 

Is it a spot check of the batch that consists of the square root +1 as is standard QC procedure?

 

If it is the case you may never even get a bolt that is HPI tested, just a spot check of the batch was tested. Do they dispose of the stressed bolt?

 

Just being the devil's advocate.  Because I think a lot of this "Quality Mfg" stuff is BS marketing.  The only out of spec part I've seen was Stag and they are somewhat highly regarded.

 

If something is MPI I know its intact at point of mfg, and that's good enough for me.

The actual HPT is a pretty detailed process.  The fixture that the testing is done in is a specific fixture that isn't cheap to build so not many places have them.  The cost isn't substantial at about $0.50/bolt when done in volume if you can find someone who has the fixture already though.

The actual HPT is a pretty detailed process.  The fixture that the testing is done in is a specific fixture that isn't cheap to build so not many places have them.  The cost isn't substantial at about $0.50/bolt when done in volume if you can find someone who has the fixture already though.

 

The question is how many are tested per/1k.   I doubt every bolt is tested.

The question is how many are tested per/1k.   I doubt every bolt is tested.

The manufacturer will test at whatever frequency the buyer requests.  Some want a 100% test while others will specify one of every n # of bolts.

The manufacturer will test at whatever frequency the buyer requests.  Some want a 100% test while others will specify one of every n # of bolts.

 

Well that sounds pretty Bogus.

Do you know what the military requires?

HPT is a mixed bag with dubious benefits.  It's a destructive test that dramatically shortens the service life of a bolt.  This is a non-issue for the military which specs a 3K round service life for the M4A1 bolts.

I would bet that they test 100% of bolts given this policy. They also play on a different scale than you and I.

 

C