Dimples on receiver and bolt

[imarangemaster]

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.

Information

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[USGI]

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

[Mikey51]

I have always understood it was a simple "Hardness Test" of the metal, done with a pin punch, but I may be wrong ? ?. Mike.

[INLAND44]

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.

[Patrick Chadwick]

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?

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

[imarangemaster]

Thanks, now I feel better. My dimples are "on top" of the finish, so it is not a repark.

[Bruce McAskill]

Proof testing if I remember right.

[Bill Ricca]

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.


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.



That Hardness Test punch mark has been a myth for years. Not on the Bolt, or Receiver.

[jmoore]

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.)

[imarangemaster]

Proof testing it is! That makes sense, as the barrel also has the small P that Bill describes. Thank you Mr. Ricca!