Headspace


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Blakenzy
March 29, 2010, 02:54 PM
Is there such a thing in revolvers? If so how do you determine how much is too much?

I was looking at a revolver loaded with empty cases from the side and when holding it up against light noticed that there is space between the base of the cases and the ... what do you call where the firing pin exits from in revolvers? breech face? I'm pretty sure it gets slightly larger in the battery or firing lockup position as the cylinder moves forward slightly. I understand that if there were no space back there then the cylinder would have a hard time spinning, but what's the upper limit on this clearance?

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rcmodel
March 29, 2010, 03:04 PM
Yes, revolvers must have a slight amount of extra headspace to allow room for the cylinder to turn freely. Also to allow for minor variation between case rim thickness due to manufacturing tolerances.

With a fired case in the chamber and the cylinder held fully to the rear, you should have about .008" -.012" (.010" optimum) gap between the case heads and the Recoil Shield (firing pin hole on the frame).

At the same time, you should have about .004" -.008" barrel cylinder gap.
End shake should probably not exceed .002" - .004".

You use a cheap automotive feeler guage set to measure the gaps in thousandths.

BTW: Hold a loaded revolver cylinder firmly in place and shake it.
You will hear the rounds rattle back & forth in the chambers.

rc

Ed Harris
March 29, 2010, 03:25 PM
With the gun in a empty, static condition, hold the cylinder to the rear and slide the thickest feeler gauge blade that will fit between the rear barrel surface and the front face of the EMPTY cylinder. Insert the feeler gage completely through the gap so that it protrudes out both sides of the cylinder. The largest gage in which you can pull the trigger through without noticeable resistance is called the "PASS" gage. The next larger gage size in which you feel resistance and cannot pull the trigger through double action is called the "HOLD" gage.

Build spec for new revolvers prior to proofing is PASS .003 and HOLD .006.
After proofing and function test the HOLD spec may not exceed 0.008

To check headspace a 0.060 max. cartridge gage is inserted between the recoil shield and the cylinder. Repeat the front gage measurement holding the cylinder forward with the rear gage in place and insert the thickest blade that fits with minimal friction. Subtract the last measurement from the first one. This is the end shake. This measurement should not exceed 0.002. If headspace is too tight, the case heads may drag on the recoil shield and bind cylinder rotation. If headspace is excessive, you can get light strikes, misfires and/or protruding primers.

If the HOLD is is less than 0.003 the cylinder will drag on the barrel when it gets fouled from shooting.

If the PASS gap exceeds mean assembly tolerance of 0.006, in the .38 Special you can expect to lose 10 f.p.s. velocity for each 0.001 increase in gap.

If PASS exceeds 0.010 the gun should not be fired with jacketed ammunition as you may stick a bullet in the barrel. Above 0.012 the gun should not be fired at all unless the barrel is set back and cylinder end play adjusted not to exceed 0.002

Blakenzy
March 29, 2010, 03:47 PM
Thanks

rcmodel
March 29, 2010, 04:11 PM
Keep in mind that various manufactures have different acceptance specs.

What would have been badly & unacceptably out of spec on a new S&W or Colt 50 years ago seems to be the norm on a lot of brands you can buy today.

They don't make'm all like they used too, that's for sure.

rc

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