Some ARs advertise the barrel is high pressure tested (HPT) and magnetic particle inspected (MPI). IIRC, some advertise those same tests for the bolt carrier group and the bolt within the group.
Do performing these test give any benefit? If these tests are performed, do they weed out anything that would have led to a much shorter service life or the risk of catastrophic failure somewhere along the line?
They weed out any material defects. Like you say… a barrel could burst if a crack in the steel wasn’t identified. Maybe lysander can shed more intimate light on the rejection rate and such.
It’s part of mil spec to keep parts suppliers honest, not required by a good company who tightly controls their raw materials. Knights Armament does not do HPT and IIRC has stated it cuts bolt life in half, they also warrant their bolts for +20,000 rounds.
American Mediocrity and Contracts awarded to minorities! I remember this crap in the 90s with Sanchez and Coopers mags. As I understand it they would give the magazine tooling to various brown people so they could make money screwing up the military’s magazines. American stupidity at its finest.
Some (very few) do all components. Most batch test. Primary reason is for liability and it also serves as a good marketing tool.
First off - MPI without HPT is pretty pointless.
If a parts is stressed (HPT), the weak areas will open up and become visible to some form of penetrant inspection. If the parts is never stressed the weal areas will never become visible.
Second, the point stressing the parts is some stuff is just not visible from the outside, (see Figure 1).
And third, the idea that a single 120% stress test will some how hurt the part is laughable. That’s not how that works. Steel has a fatigue limit, if the stresses are below this limit, it will not fail due to fatigue. Small arms barrels live in this range. M16/AR bolts are subject to low cycle fatigue. This means the stresses in the bolt under normal operation exceed the yield strength in some areas, and eventually cracks will develop. Measured life expectancy of bolts (military bolts) has shown the first crack can show up as early as 5,000 rounds, and some bolts will last over 20,000 rounds. That one loading that was 20% higher than the average is not going to shift that huge uncertainty zone enough to be noticeable.
(If you honesty believe that an HPT will shorten the life of the part, don’t ever go to your doctor for a medical stress test.)
. . . they also warrant their bolts for +20,000 rounds . . .
Please, provide a link to this. I have heard this over and over but their website states something completely different.
https://i.imgur.com/OKDbHvO.png
Figure 1
There is only one place in the world to get Carpenter 158 steel - Carpenter Technologies Inc. I do not know how much tighter you can get on your raw materials . . .
Yet, many good companied feel that there is value added to HPT/MPI their bolts, and still manage to price them competitively at $60 - $80, only $10 to $20 more than the non-HPT/MPI bolts.
“. . . penny-wise, pound foolish . . .”
HPT absolutely shortens the life of the bolt, and this was explicitly covered in a NDIA briefing.
What it does get you is assurance it will work out of the box, at the cost longevity. Probably a good idea if you want to be able to spit out a TDP to any vendor and have confidence in a QA/QC process.
You will see bolts for GPF contracts maintain the legacy HP/MPI and some to other units not specify this.
Which NDIA Brief?
Please link.
I thought knights batch HPT their barrels and bolts. They would take certain percentage of a lot and test those and then throw them away.
I think Snow Colt skips HPT, too, don’t they?
I wonder if car companies over heat the brakes on ever new car to make sure it can handle normal heat, or if aircraft companies subject the landing gear of every new airplane to an impact higher than it ever would in real life to make sure it can handle a hard landing?
I understand the wisdom, kind of. It just feels like there has to be a better way to it. Like just get material that’s really good and quality controlled? Don’t machine it too fast, ect.
Maybe make the cam pin hole smaller and round the locking lugs? I realize after I typed that, it’s done by a company already. Cross comparability is an issue. It’s too bad it wasn’t the original design.
Yes, as a matter of fact, certain life-limited components are proof loaded and inspected to verify integrity.
Turbine blisks, one-piece compressor rotors, hydraulic cylinders, pressure vessels, landing gear struts
On the ground based side of things, cranes are proof loaded above the maximum certified weight on a regular basis.
If the service load was close to the maximum allowable load, you might have a point about the proof load being damaging. However, stuff like bolts have a substantial safety factor in the design, so the proof load keeps even the proof loading from being damaging.
EDIT:
Machining at the correct speed does not preclude internal material flaws.
And last: Making the cam pin hole smaller just moves the weak point from the bolt to the cam pin.
When I was working on Minuteman Missiles in the Air Force decades ago the hoists that we used to raise the Nukes off the missiles were marked, *Proof Load Test at Twice the Rated Capacity. * That entire quote was painted on the hoists.
I dont care who makes it, raw material flaws can and do happen, and process control is a numbers game to minimize chances while remaining profitable.
I am still waiting for the answers to these two questions:
It has been claimed multiple times that Knight’s guarantees their bolts for 20,000 rounds, where does Knight’s state this?
Can anyone link to a technical report that states that proof testing measurably reduces part life?
On their website.
https://www.knightarmco.com/our-products
Under SR-16:
The heart of the product line is the E3 bolt, a proprietary design with a 20,000+ round lifespan.
That is not a guarantee, that’s advertising.
Neat.
This is what they actually state in regards to their products:
Knight’s Armament Company & KAC / KMC Limited Warranty:
Products and firearms are warranted to be free from defects in material and workmanship. This warranty shall apply to the original owner for a period of 1 year from date of purchase or delivery. Any such defects of which KAC / KMC receives written notice within 1 year from the date of purchase by the original owner, will be remedied by KAC / KMC without charge.