Barrel lockup fore-aft play in slide...

[Gliderguy]

Does anyone know a spec for maximum allowable fore and aft play for the barrel lug in the slide ejection port? My PM40 seems to have about .006-.007 I am measuring this with a cheap dial caliper that has graduations to the nearest .01 and am getting a bit more than half a graduation. I am sure it is more than .005 and definitely sure it is less than .009

I do have a tiny amount of peening on the barrel lug where the lockup occurs:




Probably a non issue but was wondering if anyone else with the proper tools would mind measuring theirs. I had the barrel and slide only (no recoil spring assembly) together, took a measurment from the barrel crown to the breech face with the barrel pushed as far forward in the slide as possible, then watched the dial as I pushed the barrel hard against the breech face to see how much movement. I did this multiple times and get pretty close to .007 difference, maybe a scosch less.

I probably would not care as the gun shoots good, In fact I cannot get the slide to hang up anywhere during the chambering of a round while riding the slide slowly forward, even when the extractor is clipping under the cartridge rim. Only have a few hundred rounds during my ownership of the pistol, but was purchased used and the serial number indicates the gun is around three years old. I just keep hearing how tight new Kahr pistol are and either I got the one with all the tolerances stacked in the loose direction or it actually has been shot a bit by the previous owner.

[jocko]

or maybe guys saying kahrs are tight might not even be referring to what ur referring to. I always felt when they stated tight it was due to the extreme difficulty of hand racking kahrs, therefore the tight phrase comes to mind. I often relate to how easy it is to rack my G19 and I don't consider it a loose gun by any means, just so so much easier to hand rack, than my kahr, but I failed to take into consideration the differenc ein recoil springs and the poundage difference of each , which I think the Glocks are around 13# where as kahrs are around 18+. Just sayin.

no doubt rounds down range will loosen a gun up "somewhat" We hear that hgere alot from owners who report back after X numberof rounds tha tthe gun feels totally different, SMMOTHNESS COULD ALSO BE LOOSENESS..

[TucsonMTB]

Might be coincidence, but my PM40 also measures 0.007" using a pretty good, stainless steel, dial indicating calipers that has markings at each 0.001" on the dial.

Edited to add: No visible peening and this puppy runs well.

[CJB]

First off... you really cant get a good reading with a caliper. You can get close, but no really good accurate reading.

To correctly read things, unmount the slide and barrel, remove the recoil spring. Clean parts squeaky clean. Mount the slide in a vice. Mount a dial test indicator (the trusty Interapid is the best) to a magnetic stand, put that on the vice. Touch the barrel lug with the Interapid, and then get the movement. Make sure the indicator is close to 90 degrees from the axis of movement (a little off... wont amount to much... just get it close to 90).

~~~

A lockup of "zero" or what we called OTN (on the nuts) is not really desirable. It hinders function, and is prone to stoppages due to dirt. Bullseye shooting... yes, .000, defense? Give it some room. Anyplace from a few .001's (maybe 2-3) to about that .007 mark is ok. Much more than that... the slide moves backward, slams the hood's front, you get peening. A little movement, and the slide can't move that fast, but with more room to move, the slide has a chance to build up more speed, and hammers harder onto the hood - making peening worse.

Can't tell ya what the Kahr spec is... but all three of mine "feel" like they're about .003ish - human hair thickness of play.

[Gliderguy]

Yeah, CJB, would do that if I had access to that kind of equipment, but all I have is this el-cheapo dial caliper. I suspect I am barking up a dead end tree, but posts have been started for much less reason I know a machinist in my hometown and he would probably give me a really accurate number, I will be heading that way in about two weeks and may bounce the request off of him.

Peening vs amount of free play is exactly what I am wondering about, the Kahr barrel/slide lockup bearing area is pretty small compared to other pistols I have had, hence my curiosity of how much slop is too much.

I would bet I could fire several thousand rounds and it would look exactly the same as my original pictures. Call it the paranoia of purchasing used since I can't know the entire history of the weapon.

Tucson, thanks for showing that mine is not an extreme example.

[Gliderguy]

CJB, I would suspect an OTN lockup would in fact lockup with about a magazine of ammo downrange to heat the barrel up a bit, I am sure the slide would be running cooler.

What you are describing is basically setting up like checking runout on rotating parts, right? I am sure my machinist has a runout gage.

[TucsonMTB]

Tucson, thanks for showing that mine is not an extreme example.

Well, as CJB points out, don't put too much faith in the 0.007" number that I thought I measured with my dial indicator.

I just messed around for a few minutes with small "feeler guages" cut from various pieces of paper here in my office. The two pieces of the thinnest paper measure 0.004" with my caliper, stacked together, and slip in and out with no resistance, while holding the barrel forward. A single piece of paper that measures 0.006" won't move at all. I is clamped hard enough to make it necessary to force the barrel hood down to get it out. I finally found a couple pieces of paper that would measure about 0.005" and they drag somewhat while pulling them out, probably more than I would consider a good measurement using a steel feeler gauge.

Anyway, I am now "guessing" mine is really 0.005" or slightly less, with both parts at room temperature.

Neither the barrel or the slide show any signs of peening, even after shooting a couple dozen rounds of fairly hot Double Tap brand .40 S&W ammo as well as over a thousand rounds of more normal Winchester Ranger.

Hey, CJB, is 0.005" two RCH's?

[CJB]

The RCH is a tough beast to measure. First you gotta FIND one, a hard thing to do these days!~

Next, they are flat - hence the tend to twist in a haphazard helix manner.

So, across the flats you're probably about .0025ish, making it .005ish for two. Across the width... you're maybe triple that.

Don't ask me how I know.

[TucsonMTB]