Head space


June 22, 2010, 09:11 AM
My brand new 686 is having light strike problem. One out of every 20 rounds doesn't go bang. Therefore, I took an automotive feeler gauge and measured the distance between the back of the cartridge and the frame of the revolver. The thickest gauge that fits is 0.014 inch. Is this too much?

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June 22, 2010, 11:11 AM

Have you checked the strain screw on the lower front part of the grip frame?

Ed Harris
June 22, 2010, 12:29 PM
On a .38/.357 revolver you can stack feeler gages to simulate rim thickness, and with the chambers empty 0.060 should enter between the rear face of the cylinder and the face of the recoil shield, and 0.066 should not. Or if you have multiple revolvers or want something to check used guns prior to purchase, get the Brownell gage http://www.brownells.com/userdocs/learn/Inst-200.pdf

Cylinder gap should not exceed 0.008" with the GO - 0.060 gage in place. End shake should not exceed 0.002, subtracting the gap with rear gage in place to the maximum that can be inserted between barrel and cylinder holding the empty cylinder to the rear.

June 22, 2010, 07:06 PM
Yes, strain screw is tight. This is a new gun.

I stack the feeler and 0.066 fits, barely. So what should I do now?

June 22, 2010, 07:55 PM
Call S&W

June 23, 2010, 12:21 AM
Could be the hammer block lever is timed improperly and the hammer is hitting it on the way down.

June 23, 2010, 05:04 AM
I edited my original post. After checking my fired brass again, I realize the indention on the primer is nice and deep for both single and double action. The light indention problem that I mention in my origin post was only present for the first box of ammo (Monarch brand). After that, the indention is nice and deep as it should be. So, the situation now is this: the firing pin is usually making a nice strike on the primer but occassionally (1 out of 25) it would make a light strike. I wonder if my problem is partially due to not totally seating the primer. I still need to contact S&W about the headspace.

June 23, 2010, 09:22 AM
I found an article about headspace. It claims max headspace is 0.074.


For many years, the firearms industry group known as the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) set standards for critical dimensions for firearms and ammunition. According to SAAMI, the minimum acceptable distance, or headspace, between the rim seat on the cylinder and the breechface for a .38 Special revolver is .060 inch. The maximum acceptable headspace is .074 inch. Thus we have a tolerance of .014 inch between minimum and maximum headspace. As long as a revolver meets these standards, it is considered to have proper headspace.

June 23, 2010, 09:34 AM
I wonder if my problem is partially due to not totally seating the primer.

That's what I was thinking. OTOH, a stock action ought to light them off pretty reliably.

This is a new gun.

Original Hogue rubber grips? It's possible the screw is a wee too tight after someone re-installed them, in which case, the grips may be pinching or rubbing the mainspring a bit, effectively lightening the hammer strike. Easy enough to check.

June 23, 2010, 11:30 AM
If the primer indentation is nice and deep, and the primer isn't firing, then it's the primer. Lately, with all this run on ammo and components, quality isn't what it used to be. Or, the primer may not have been seated all the way to the bottom of the primer pocket. Get a Hornady hand priming tool.

Don't worry about your headspace. It is fine. .014" clearance is less than what I have on my PC 625-7.

Jim K
June 23, 2010, 08:11 PM
It sounds to me like the ammo. When the primer fires in the normal manner, it flows back to the breech face and around the firing pin and gives that deep look we recognize immediately. But when the primer doesn't fire, it is just dented. The dent is just as deep, but the pressure inside the primer isn't there and the primer does not flatten back out and flow around the firing pin so the dent appears shallow. You can measure the depth of the indent, but it sounds to me like dead primers. (FWIW, I have seen reports of problems with Monarch ammo, but others seem satisfied with it.)


The Lone Haranguer
June 23, 2010, 09:20 PM
I wonder if my problem is partially due to not totally seating the primer.
This is very plausible. If the primer is not fully seated the hammer blow will be cushioned because it expends energy driving the primer deeper into the pocket. It might fire on the second go-round. I've had this happen with percussion caps.

June 23, 2010, 09:53 PM
If you're not setting off 1 out of 20/25, something is up. If it's not the ammo, and if the spring is not giving you light strikes, you need to look at the firing pin.

If the same type of ammo is being used, you may have sporadic high primers, or bad primers--primers that may have been exposed to something.

I always measured my headspace with an empty shell in place, and then anything more than .009 was not acceptable. (My Rugers were always in-spec, but I had several S&Ws that had excess space, and they often had problems.)

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