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This is a really dumb question, especially since I used to know the answer many years ago, but "Old Timers" has scrubbed it from my brain. What is the meaning of the small dimple on the front of the sight bridge and the bolt head. It seems to me it has to do with rebuilding or servicing, but mine is original and has the marks. Thanks in advance. Inquiring minds want to know.
Off the top of my head, it indicates those parts have passed proof testing. I bet the dimple on your new Inland receiver is "on top" of the finish, showing it hasn't been reparked. - Bob
I have always understood it was a simple "Hardness Test" of the metal, done with a pin punch, but I may be wrong ? ?. Mike.
That's it - verification of hardness with a punch. I have no idea how they controlled that process unless they had a machine for it. I mean, how do you hit a punch with a certain diameter, hardness and tip shape with a hammer, to deliver an exact and repeatable force? To do that by hand is impossible, but perhaps it didn't need to be as technical as I'm trying to make it. Maybe you just take a center punch and ball peen hammer and give the object a medium shot, not like you're trying to drive the punch through it. After doing a few hundred bolts, I imagine you would see and feel a difference if you hit a 'soft' one.
Thanks, now I feel better. My dimples are "on top" of the finish, so it is not a repark.
Proof testing if I remember right.
Hardness testing on the bolt and receiver is incorrect. The punch marks are applied when the bolt and receiver have passed the High Pressure Test. The barrels were marked with a P.
Each rifle, bolt, barrel, and/or receiver had to be tested with a High Pressure Test Round. It applied to the Garand, Carbine, M14, 1911A1, etc.
Below shows an M14 NM Manual and the punch marks.
https://www.milsurps.com/images/impo...ec_Marks-1.jpg
Each proof fire had to be done under safe conditions. Sometimes the actions were fired remotely from another room, depending upon how the contractor set it up. The safety set up had to be approved. On the left is a carbine, on the right is a carbine barrel. Both are show being High Pressure Tested.
https://www.milsurps.com/images/impo...rb_proof-1.jpg
That Hardness Test punch mark has been a myth for years. Not on the Bolt, or Receiver.
A Rockwell "C" series hardness test uses a well defined angled punch that is preloaded and then loaded again to a specific force, leaving a very small indentation, the depth of which is measured by the machine using the punch shaft displacement into the metal before it's removed from the machine. (There's some older methods that measure the dimple by other means, but they wouldn't apply here, either.) Regardless, a Rockwell dimple is VERY small and regularly shaped, not at all like whats found on bolts, etc.
The hardness test would also not be accurate except on surfaces that are solid from the indentation site to the bottom of the test piece. You want no holes in the area! Some areas on a bolt lug would be OK as they're solid. But a bolt body has at least the firing pin hole bored through it, so that area is no good.
(BTW, I'd estimate a Rockwell punch mark to be maybe 0.010 0.015" max. in diameter on normal action parts. So small it's useless for an inspection marking.)
Proof testing it is! That makes sense, as the barrel also has the small P that Bill describes. Thank you Mr. Ricca!
There are some barrels that have the P and the punch mark inside the P. I guess those were spare barrels that were tested and P marked. Then assembled in a rifle tested and the punch added to the P. Just a guess.
BTW this shows why you can never believe advertisers calling they commercial barrels "Mil Spec". Many do not know what Mil Spec means. No commercial barrel has ever been tested with a GI High Pressure Test Round. That ammunition is almost impossible to produce, unless you have a large GI contract and get the ammuntion from Ordnance.
On the fun side look at the M14 Picture and see the military description of the punch mark. That word cannot be used on many forums "Prick Punch Markings". I could not post it on some forums due to the nasty word.
That is silly as it's name is "Prick Punch". Any machinist knows and has them. I guess you can't call a donkey an "***" either. Got edited out.Quote:
Originally Posted by Bill Ricca
Holy moly, what are we becoming.
Jim
Many Thanks Bill ! Your input as usual, is very much appreciated. Mike.
This may sound like a dumb question, but what's the difference between "High Pressure Testing" and Hardness testing. Doesn't the "high pressure test" conclude that the steel is hard enough to withstand the "high pressure prick"? Or I guess I should say "Prick Punch".
Quote:
Originally Posted by INLAND44
With a specialist machine, as you guessed. A local gunsmith, who is also a qualified toolmaker, even has one!
Here are some examples:
http://www.hardnesstesters.com/Products/index.aspx
Quote:
Originally Posted by ABPOS
High pressure testing applies a force over a considerable area, but remains within the elastic limits of the material. A proof round, for instance, is not supposed to permanently distort the chamber! But the old CUP measurement was derived from the measured distortion of the copper block. The intention is to check that the material under test will indeed not be stressed beyond the elastic limit in the intended appplication.
Hardness testing measures just what that elastic limit is, by applying a much higher force than a pressure test, but to a very small area that is indeed distorted beyond the elastic limit, and the hardness figure is derived from measurement of that distortion. If the pressure produced on the tiny spot of a Rockwell tester were to be applied all over a chamber, the chamber would not just distort, it would explode.
After the high pressure round is fired, the bolt, barrel and/or chamber are checked for cracks and other damages. The round bolts from the 1960's that had the M marking were often advertised as Winchester (upsidedown M). The M meant the bolt was Magnetic Particle Tested or Magafluzed and no cracks were found. The punches and/or the P means the items were tested and passed.
I have the pressure of the different Test Rounds and the pressure is not the only difference. The types of powder change the pressure flow so the pressure is different from standard ammunition as the bullet travels though the rifling. I do not remember each one.
That high pressure test will show that normal ammunition will not hurt the tested item.
Hardness testing is usually part of production and will see that the item will not bent, break, or have other problems.
I think the punch mark on a carbine barrel was NPM's own proof mark, any "P" would have been applied by the barrel's maker after proof firing (NPM didn't make their own barrels).Quote:
Originally Posted by Bill Ricca
That makes sense. But for some reason, I'm getting the idea that the "high pressure test" is an actual round being fired out of the chamber. But that doesn't seem to make sense with the dimples or "pricking". I'm probably just reading it wrong and am confused. I can't really see bills pictures all that well, other than it looks like the rifle in some kind of contraption. It does look like it might be allowing something to come out the muzzle, clear of the device though. But why would that leave a dimple on the oustide of the bolt and receiver? But again, I'm probably just not understanding it correctly. Because there seem to be more evidence of something actually striking those areas. Right?Quote:
Originally Posted by Patrick Chadwick
---------- Post added at 12:11 PM ---------- Previous post was at 12:10 PM ----------
Or maybe it's both?
After the rifle has been test fired with the high pressure round and has passed inspection, they place the punch marks on the bolt and receiver to indicate such.
- Bob
OK, got it. Are bolts marked anywhere else with any letters typically?
You know what? I knew that but had forgotten it! Various steel parts on many U.S. guns did have hardness tests done the same way - I guess I just mixed it all up together