What Is Criteria for Light Primer Strikes?

[DB]

This morning I was looking through some once-fired brass that I have and noticed that I had a great range of different primer strike marks on brass fired through my different handguns. Some .45ACP and revolver rounds had large and deep strikes, whereas some of my 9mm and .40 cal brass looked like it was barely nicked at all. Getting the idea from an internet site, I setup a fixture with a dial indicator to measure the depth of the primer strikes. The results ranged from as little as .004"-.005" (on 9mm fired through a Ruger P95DC) to as much as .022 for brass fired through a K-Frame S&W .38 revolver (older type with firing pin on the hammer). I don't recall that I have had a failure-to-fire problem with any of my guns. However, I also don't want a misfire to happen at the worst possible moment. Is there any industry or gunsmithing standard measurement or criteria on what constitutes a light primer strike?

 

[esheato]

Is there any industry or gunsmithing standard measurement or criteria on what constitutes a light primer strike?

Not that I'm aware of.

FYI, I don't start worrying about it until the gun fails to fire. Shallow or deep, doesn't matter until the tool doesn't do what it is supposed to do.

Ed

 

[JesseL]

If i was trying to do a meaningful comparison between primer indentations, I'd do my measurements on primed brass (live primer, no powder, no bullet) rather than fired rounds. The varying degrees of pressure inside different rounds could reshape the dimples before you ever see them.

This would seem to jibe with your results, ie your higher pressure rounds (9mm, 40 S&W) have much shallower dimples than the low pressure ones (38 special, 45acp).

 

[gamachinist]

JesseL makes a very good point,and I would like to throw another variable into the mix:
Headspace.

Most revolvers have better controled headspace than many semi autos.

Another variable it the diameter of the pin tip,since most 9mm's have smaller pins than most 45's.

But presure is probably one of the main differences.

 

[Azrael256]

Usually, the test for primer strike is not depth, but reliability of ignition. Basically, if the gun pops a CCI primer with regularity, it'll fire anything.

JesseL is right about pressure. There are guns that are actually cycled by primers backing out. A few gun designers learned the hard way that primers pop out with enough force to damage the gun. Even the venerable 1911 design had to be reworked to deal with this issue. If it backs out with enough force to damage the breechface with a low-pressure .45ACP, there's more than enough to flatten out the strike on a 9mm or .40 primer.

 

[RecoilRob]

A friend showed me an '06 case fired in his Garand....that lacked a flash hole. Cycled the action and the extractor actually pulled a section of the rim off! He thought it was a squib at first but the bullet was, obviously, still in the case lying on the ground.

 

[mete]

There is , in fact , criteria for firing pin strike ,in inch/pounds ,and I know I wrote it down but can't find it !!! There is also firing pin protrusion and tip radius. The important thing is - does it pop the primer.

 

[DB]

Thanks! I've learned some things from you guys that I hadn't even thought about. I guess one of the best ways to see if I'm really getting light primer strikes with a particular gun is to run a bunch of CCI primed ammo through it. Proof will be in the pudding.