How do I measure headspace on my model 25? All I have is feeler gauges. What is acceptable measurement? Thanks in advance.

If its a 45ACP it headspaces via the moon clip.
Are you talking about cyl gap?
What's the issue driving the question?

I'm just measuring b/c gap, end shake and basically checking everything out. I always check new to me guns before firing very much. I know what my other revolvers measure and what the acceptable ranges are for the headspace, but I don't know how to measure the headspace or what it should be on this one.

SWAG* here: measure thickness of the mood clips and add that to the usual 0.060-0.068 of most S&W revolvers.

*(Scientific Wild *** Guess)

If it is a 45ACP and not a 45 Colt, it headspaces on the cartridge case mouth and the gun can be fired without moon clips. The moon clips are a convenience, not a necessity. Look into the cylinder holes from the back end and look for ridges against which the cartridge case edge rests. Or you can simply place a cartridge into the cylinder hole. It should stop short of the rim contacting the cylinder.

The OP needs to confirm the M25 being either 45ACP or 45 Colt to receive the correct answer.

.

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Waiting for the break of day

A simple method is to load the cylinder with fired or empty cases, close cylinder. Measurement from case head to recoil shield at firing pin with cylinder pressed back, it should be around .006 +-.004 Less isn't a problem unless you should run into a batch of cases with thick rims (or in the case of a moonclip gun, thick clips) or a build up of crud under the extractor star.

But it is best to check end shake first. Press cylinder forward and measure barrel to cylinder. Then press cylinder to rear and measure. Subtract forward from rear. Should be ,001-.002. Adjust this to spec. via shims or stretching yoke tube, then check head space.

Excess endshake will cause variation in head space (cylinder forward or cylinder back), timing issues etc. Head space is controlled by the little round area in center of ratchet. It could be tightened by carefully stoning that area. About the only way to open head space would be to replace extractor star, or removing material from back of cylinder itself (this could also cause you to need to ream chambers same amount) or stoning recoil shield. I have never had this problem on any of my revolvers or on the dozen or so cylinders I have replaced. I believe the factory must do a very good job of keeping the extractor with in tolerance and the recoil shield flat and square.

Thanks for your replies. It is a m25acp.

Rear gage (headspace) specs for the .45 ACP are .090" - .094". Headspace is measured with feeler gauges inserted between the breechface and the rear of the cylinder, in the area of the firing pin/hammer nose bushing.

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Ret. LE, FA Instr, S&W Armorer

Did it both ways. Steelslaver way- .006, Armorer951- .09. Looks like it's good to go. Thanks a lot.