Cylinder Gap on Old No Dash 686

[Hapworth]

...

QUESTION: Would replacing the barrel with a new one, eliminate the shortening of the extractor rod and center pin? I am assuming that the current widened b/c gap is a result of frame stretch, right?

The most recent trip back to S&W(last month), additional shims/washers were installed to eliminate the then current endshake. Wouldn't setting/shortening the barrel eliminate the need for such "added" washers and self-correct the loose endshake?

Frame stretch can (among other factors) contribute to increased b-c gap and endshake, but generally the issue is the yoke barrel compacting with use and the corrective is stretching it; if S&W is using shims for endshake now I'm surprised -- that never used to be the factory fix.

As to diminishing returns in muzzle velocity past a certain b-c gap, I alluded in an earlier post to a half-remembered test that indicated after a certain amount of gap you do begin to see an increase in MV drop closer to the largest one seen in the .000" to .001" drop versus the rest of the increments out to about .010", but it was past .012" that this started to happen, if I recall and wasn't extreme -- you're well shy of it and it really isn't a pressing matter in your situation anyway.

More on diminishing returns, broadly...

Mounting a new barrel may or may not get you out of shortening the extractor rod and center pin, but the barrel face would likely still have to be cut and trued for ideal b-c gap.

And even then, there's a lot more that goes into addressing all the factors affecting bullet performance. What if you barrel swap (which needs to be done by someone with the right know-how and tools, at cost) and it turns out b-c gap wasn't a significantly affecting issue in your revolver after all? Is it timing? Is it charge hole diameter? Shoulder cut? Slow rifling? What if the new barrel introduces problems the old barrel didn't?

The revolver is a system. The factors affecting ballistic performance are throughout. Chasing one at a time when there isn't a glaring problem is likely a waste and may be counterproductive.

And it's still not clear exactly why? If this 686 is to be a load tester, you don't need it to wring every last bit of performance out of a given round, you just need to establish its baseline by running different loads and collecting the data, then compare other guns against it.

But if for some reason you insist, don't chase single (and variable affecting) factors like b-c gap -- send the revolver to a top-shelf revolversmith, explain to them your intended use for it, and ask the revolver be blue-printed stem-to-stern. That's the only way to address ever significant element in the revolver that influences bullet behavior. Of course, then you'll have more into that 686 and it'll be nicer than the one you're trying to preserve, and you might have to go buy another shooter for testing...

 

[Forrest r]

The only time I ever cared about the bc gap & velocity was when I was using/testing snubnosed revolvers. +/- 50fps is huge with sd loads that don't have a lot speed to begin with due to the short bbl's.

The potential for pre-mature wear and lack of accuracy was always my main concern when the bc gap got over 7/1000th's.

Look at it this way, draw a strait line and then another line at a draw a 45* intersecting the 1st strait line you drew. The further away from where the 2 lines crossed, the larger the distance becomes. Anything under pressure goes to the least point of resistance. You hit the loud button, bang the bullets off to the races. The cylinder also moves/jumps to the furthest point it can. This mis-alignment is negated by the angle of the forcing cone. If you look at the picture of the bbl I took off that 586 you can clearly see that not only is the forcing cone egg shaped, it is also starting to wear on the left side. The side that the cylinder swings open.

The longer the distance of the bc gap, the more distortion the bullet will have to withstand. Coupled with wider strikes on the forcing cone.

Don't know if you ever bothered to measure the length of the cylinder itself. They have a +/- of 6/1000th's to them. A simple cylinder swap could tighten the bc gap up. The last revolver I swapped out the cylinders in was a charter arms undercover. Doing so gained 40fps from a 38spl p+ reload using 158gr cast hp's and power pistol. The bc gap is now a tight 3/1000th's

 

[HorizontalMike]

Well... the only loads that I am really interested in are .38+P and upper end .357 Magnum loads in a 4 inch barrel. The .357 Magnum loads are for a 158XTP only. The .38+P loads (currently) look like 125gr Golden Saber, 125g and 135gr Speer, and 125gr XTP. I have found that I cannot get sufficient MV out of my 2-inch small frames for 158XTPs (no expansion).

That said, I have no problem using either my 2-inch or my 2.625-inch revolvers for that. Those lighter bullets are/were designed for lower MVs. Once I get those loads worked out, then the 4" 686 will only be for .357 Magnum loads of N105 at the top end, or about 1350-1400fps (out of my better 686). At least now I understand, at least roughly, how much MV loss to expect with the larger b/c gap.

 

[HorizontalMike]

The only time I ever cared about the bc gap & velocity was when I was using/testing snubnosed revolvers. +/- 50fps is huge with sd loads that don't have a lot speed to begin with due to the short bbl's.
...[snip]...
Don't know if you ever bothered to measure the length of the cylinder itself. They have a +/- of 6/1000th's to them. A simple cylinder swap could tighten the bc gap up. The last revolver I swapped out the cylinders in was a charter arms undercover. Doing so gained 40fps from a 38spl p+ reload using 158gr cast hp's and power pistol. The bc gap is now a tight 3/1000th's

So, as I understand it, my actual b/c gaps are pass .005" and pass .009". So my MV loss is/should be about 40fps between the two barrels since the difference between barrels is .004"...

Closer than what I was originally fearing. Boy that Wilson Combat spring and pin kit really lightened up the SA to almost a hair trigger, and that was with the heaviest 14lb rebound spring. The DA is pretty much now how I like it now.

 

[WR Moore]

QUESTION: Would replacing the barrel with a new one, eliminate the shortening of the extractor rod and center pin? I am assuming that the current widened b/c gap is a result of frame stretch, right?

Don't jump on unlikely causes when simple wear & tear is the most likely cause*. Don't confuse the various measurements and assume that if one is corrected, the other has to be excessive.

When your revolver was first assembled, it had acceptable headspace (0.060-0.068 inc), endplay and barrel cylinder gap (.004-0.008/0.012 inch depending upon factory tolerance spec). Now through wear, your revolver developed excessive endplay which was reset by the factory to an acceptable point. Previously, the cure for that was stretching the yoke barrel. I'd be surprised if they went to shims.

At that time, the headspace and b/c gap would have been checked to make sure they were within current tolerances. It would appear yours is considered within tolerance. Alternatively, the b/c gap was a little sloppy originally, they saw no reason to fix what ain't broke and/or the tolerance may have changed.

Now, yes, installing a new barrel would eliminate shortening the extractor rod & center pin. However, depending upon what the factory now considers to be acceptable b/c gap, you might not see any improvement (unless you send it to the Performance Center and specify desired headspace & B/C gap). On top of that, you'd be shelling out the cost of a new barrel plus labor and possibly plus other parts. It'd probably be a wash between reworking the gun with the current barrel and fitting a new barrel.

Why don't you just shoot the cotton picking thing? OOPS! it appears that's what you've decided.


* One employer issued the 681 and all training, qualification and other shooting was done with full power Federal 125 gr JHP Magnums. The guns got loose, would eventually get factory rebuilds but, SFAIK, we never stretched a frame despite tens of thousands of rounds per gun. OTOH, overzealous cleaning caused the replacement of several cylinders due to excessive wear of the face.

 

[HorizontalMike]

Don't jump on unlikely causes when simple wear & tear is the most likely cause*. ...[snip]...
...[snip]... It'd probably be a wash between reworking the gun with the current barrel and fitting a new barrel.

Why don't you just shoot the cotton picking thing? OOPS! it appears that's what you've decided.

...[snip]...

That goes without saying, again. This new-to-me-686 was purchased 2nd/3rd/4th/etc. hand. Trying to counter/control/understand the level of abuse/use this gun had in the past, will at least assist in going forward. That is the point of this thread.

BTW, shooting the Kayrapp out of it, without any measurement of parameters, IMO is an exercise in futility. In other words, if you don't know where your started then HOW do you know where you are going end up, and when to stop with "adjustments" per say?
BTW..., I actually do want to shoot the Kayrapp out of it, BUTT take adequate notes as I do so in the mean time. To NOT do that, is tantamount to waiting on the Monday Morning Paper to find out if the proverbially "you" was a wanted individual for a murderous act, if you know what I mean...

I shoot PAPER, not people. Make a point of that please... Accurate information is required at all times... Thanks...

 

[Forrest r]

So, as I understand it, my actual b/c gaps are pass .005" and pass .009". So my MV loss is/should be about 40fps between the two barrels since the difference between barrels is .004"...

Closer than what I was originally fearing. Boy that Wilson Combat spring and pin kit really lightened up the SA to almost a hair trigger, and that was with the heaviest 14lb rebound spring. The DA is pretty much now how I like it now.

Correct, you'll be in that +/- 40fps range.

Don't forget that using 38spl's in a 357 cylinder will also cost you +/- 10fps.

A simple test:
Load up fullhouse/hot 357 loads with cast bullets. Take plain white printer paper to the range along with a box to tape the paper to. Sit at a table and run 4 cylinders full of the hot loads with the box/paper on the right side next to the revolver (6" away). Put on new paper (save/mark the old paper gun side) do the left side. Switch firearms and repeat.

After both firearms are tested, take a good look at both revolvers looking for lead build-up around the forcing cone. Look at the paper for lead splatter/excessive powder marks.

No excessive lead build-up or excessive splatter ='s a tight revolver. Leading & splatter ='s the revolver is getting loose.

It takes a lot of rounds/hot loads to shoot a 586/686 loose. Back in the day I'd buy #32 (4 #8 jugs/1 case) of the ww820 pulldown powder from pat's reloading at the medina gunshow's. I'd burn that powder up in that 586 +/- 14gr at a time and more often then not the loads were north of 14 gr. That's +/- 15,000 fullhose loads to a case of powder. I never kept count of how many case of that powder I bought. But that's all I loaded/used for several years.

I do find it interesting with the comments on shorting rods/etc. When you re-barrel a s&w the most important thing is the forcing cone. More specifically the width of the forcing cone. The length of the ejector rods don't come into play. At the end of the day you're dealing with 28/1000th's of an inch when re-barreling a 686. Facing off the bbl take up most of that 28/1000th's. The rest is set with the cylinders.

Why do I have pictures of a 568 bbl in my hand? Because I bought a 4" bbl for that 568 with the shot-out 6" bbl. Put it on and ended up with a 8/1000th's bc gap. Shopped around and found a cylinder that was 4/1000th's longer then the original and put it on that 586. That 586 shot lights out with the new parts (4/1000th's bc gap) but it looked bad with the extremely worn frame. I didn't care was glad to have it back up and running. Ended up selling it to help fund a 686 competitor. Always like a 6" bbl for plinking/target work.

Another bbl change, this time a ca bulldog. Hated the high front sight!!! Why they would put a fish hook of a front sight on a pocket pistol is beyond me. Anyway picked up both of these ca pistols for $300. Note the wear/turn lines on the ca undercover. The picture I posted a couple days ago is the same revolver with a cylinder swap.

Put a longer bbl (2 1/2" vs 2") new style (lower front sight) on that buldog with a 6/1000th's bc gap.

Now that bulldog is a lot easier to carry & is a real thumper.

You should test that 686 to make sure it hasn't been shot loose. If it's tight, shoot it till it is loose.

 

[Nevada Ed]

I had an eight year old 686 that I had to send in, due to bullet frag
coming back at me.

I have been too busy with the 38 J frames and 9mm's to shoot it
but I will get to test it out soon.

Hope that "Tight" revolver does well, for you.

 

[Engineer1911]

My brain is itching from all the data and statistics.

I can predict with absolute certainty that there is greater velocity variation shot to shot in a 50 round box of ammo than there is in the "barrel/cylinder" gap variation.

Really ? ? ? Sometimes it is just fun to go to the range with guns & bullets just for the joy of shooting. Like this (quoted from Match Results):

The Pistol Challenge went good on Saturday [10-06-18] and we had fun. We had 17 shooters mostly members.

Max Possible is 300 with 30 - X

First Place:: Archer B. - 298 / 17 X --- S&W model 41 22 LR
Second Place: Frank D. - 287 / 10 X --- Colt 1911 National Match 45 ACP
Third Place: Col. Bollinger - 285 / 4 X --- firearm unknown

Engineer1911 as the only shooter who has shot in a sanctioned match, did a demonstration shoot with a 38 Spl M-15 S&W with a 2 inch barrel shooting all double action - 293 / 11 X.

 

[HorizontalMike]

Now Have 0.003in BC Gap on my 686 4in

Well, I finally got my 2nd 686 back from the LGS, after having the shoulder milled and the B/C Gap reduced/reset. I had asked for a 0.004in gap, but when all was said and done, I ended up with 0.003in Gap. All appears well IMO. Got the chronograph on 42 SA shots and then saved the chrono and shot another 8-10 shot in DA only.

The chrono results came in at 1401fps, or about 30fps faster than my other 686 that has a .005in B/C gap. All rounds were reloads on 12.4gr N105 w/158xtp.

Feels like all is well in DA even after 50 rounds. Although, I did notice a slight scratch across the cylinder's powder marks emanating from the upper barrel contacting the cylinder ever so slightly. Should I try lightly stoning this?

 

[Protocall_Design]

It looks like there's a metal burr or built up powder residue at the top of the forcing cone that would make the mark on the cylinder. You are likely to get that any time you have a tight cylinder gap, especially with any endshake. You will need to keep an eye on the endshake situation. If you get more, the cylinder will contact the barrel more.

 

[HorizontalMike]

It looks like there's a metal burr or built up powder residue at the top of the forcing cone that would make the mark on the cylinder. You are likely to get that any time you have a tight cylinder gap, especially with any endshake. You will need to keep an eye on the endshake situation. If you get more, the cylinder will contact the barrel more.

Good idea. FWIW, appears that the cylinder contacts barrel while at rest. Sliding the cylinder to the rear, I get the pass = 0.003in and hold = 0.004in

Got some 0.002in endshake bearings and an extractor rod removal tool in my cart at MW.

 

[rockquarry]

I have a 25-2 .45 ACP / Auto Rim from the 70s; standard gun, 6 1/2" barrel. I've had it a long time. Cylinder gap is .011". Muzzle velocity of my favorite cast 200 grain SWC load is about 100 fps less than that from a 5" Govt. Model Colt.

Granted, I should send it to the factory for fixing, just haven't gotten around to it. Regardless, the revolver is very accurate, even if it shouldn't be.

 

[HorizontalMike]

Great NEWS! Endshake Fixed! On BOTH 686's

It looks like there's a metal burr or built up powder residue at the top of the forcing cone that would make the mark on the cylinder. You are likely to get that any time you have a tight cylinder gap, especially with any endshake. You will need to keep an eye on the endshake situation. If you get more, the cylinder will contact the barrel more.

Here you go, GREAT News!... Got my cylinder shims in today, from MW. Then I went to work...

WORKING FROM ATTACHED IMAGE:

• #2 686 was my 2nd 686 and my target for max improvement.
• Started with only ONE 0.002in shim. Ended up adding another for a total of 0.004in in shims.
• Best I can tell is that #2 686 end-shake is now less than 0.001in. I say this because max B/C is still <0.004in pulling cylinder back and pulling FORWARD, the cylinder still drags but allows the feeler gauge to slip with a 0.004in feeler inplace.
  
  THEN on #1 686:
• On #1, my ORIGINAL 686 bought in 1986, my original B/C was pass 0.005in and hold/drag 0.006in
• Added TWO 0.002in shims and eliminated all measurable endshake, without changing B/C gap
• New endshake also measures less than 0.001in

FWIW, I have not field tested these shimmed 686s "HOT", but things look good IMO. Worst case = shortening my "Range Hot Time" with the local Sheriff's Office (verbal agreement that has been working very well in this very rural County in Texas).

 

[HorizontalMike]

Cylinder EndShake set TOO tight?

OK. Got a chance to burn through a couple of boxes today in DA, testing both "now shimmed cylinder" 686s. Got blister on trigger finger to prove it...

My tightened B/C 686 shot flawlessly, with 0.004in cylinder shims.

My original "bought new" 686, just started having light primer strikes, after adding 0.004in cylinder shims. I had 3-in-a-row on one full cylinder, and then had another two later, on probably the third reload. Never had this issue with this gun. History: I had a professional LGS trigger job on it, back in 1987.

BOTH 686s showed NO measurable endshake after adding shims. I checked with feeler gauges.

QUESTION:
Is it possible that getting the endshake set TOO tight, could actually cause these DA light primer strikes?

FWIW, I looked at these rounds and they DID NOT look like any of my 686 fired rounds, EVER, going back years. These light strikes are new... I was able to fire these rounds in my other 686.

I have since removed one 0.002in shim from my original "bought new" 686, but have not been able to test-fire this 686 again today. Maybe tomorrow morning.

 

[Ole Joe Clark]

I love my 686 no dash, but haven't thought of it as being old. I only bought it new in 1987, so I still think of it as new.

Have a blessed day,

Leon