Cylinder Gap on Old No Dash 686

[HorizontalMike]

After sending off my new-to-me 686 No Dash to S&W for some service work (cylinder release sticky and for "M"), I finally got a chance to fire it and site it properly. I also took the time to take some measurements of the cylinder gap on this gun, as well as my original bought new 686 No Dash. My measurements on at the bottom of page.

I followed the advice that I found on the following website:
castboolits.gunloads.com/Excessive-barrel-cylinder-gap

Check the barrel-cylinder gap WITH EMPTY BRASS IN THE GUN!

PASS dimension is the largest feeler gage size you can insert between the barrel and cylinder, protruding from both sides of the frame window, which permits pulling the trigger six times to rotate the cylinder through a full revolution without binding up.

HOLD dimension is the size which in the same test condition as above causes the cylinder to bind before completing a full revolution, with fired brass in the gun (substitutes for "rear gage").

END SHAKE or END PLAY is the difference between two HOLD measurements, taken both with, and without "rear gage" in place.

New builds before proofing min pass 0.003/hold 0.004

After proofing max pass 0.006/hold 0.008

Customer service max pass 0.007/hold 0.009

Cylinder gap over 0.012" should not be fired as you may stick bullet in barrel.

End Shake over 0.003" stretch crane arbor then recheck gap.

If HOLD gap after over 0.009 refit with "long" cylinder.

Current specs may vary, but this is how I built them in the 1980s.

My Gap measurements

My new-to-me 686 recent GB purchase:

• Pass = .009"
• Hold = .010"
• Endshake less than .001"

Is the above too much gap at this point?
Or should I not worry about it at this stage and shoot the Kayrapp out of it?...


My original 686:

• Pass = .005"
• Hold = .006"
• Endshake less than .001"

 

[Protocall_Design]

The original at .006 is pretty much what you want. The .010 is a bit wide, but will still work fine. Probably not worth doing anything about.

 

[Dieseltech56]

I have a few 686s and they all average out around .008". One no dash I have is .005". I would say anything above .010" is concerning even though I believe .012" is allowable.

 

[HorizontalMike]

The original at .006 is pretty much what you want. The .010 is a bit wide, but will still work fine. Probably not worth doing anything about.

Thanks for responding. What I am wondering about is... Is ther any way to determine how much energy/fps is lost for each additional .001" of additional gap? I have been searching for this specific parameter, however, I have not found such information thus far...

My best chronographs/radar readings indicate at least a 250fps differentiation between the two 686s, but the data is so limited (had really hard time getting LabRadar to record data from the recent purchase 686..). Have not been able to record SA rounds through my replacement chronograph. Will attempt ASAP, since my previous DA measurements resulted in a DOA chrono....
Suffice it to say that I HATE trying to shoot DA at ~13yd, through a regular "chronograph" with a DA-only revolver... ERRgh!...

Bottom line, I realize that I am going to lose "some" fps with a wider gap, but just how much is the question...

 

[Sevens]

This is going to sound flippant and rude, and I swear, that isn't my intention. But... honestly, does it matter?

I believe it would definitely be interesting to know how much difference in velocity you might see but beyond interesting, I am left with "does it matter and if so, how?"

 

[skipnsb]

"ballistics by the inch" has some comparisons of velocity v. gap

 

[HorizontalMike]

This is going to sound flippant and rude, and I swear, that isn't my intention. But... honestly, does it matter?

I believe it would definitely be interesting to know how much difference in velocity you might see but beyond interesting, I am left with "does it matter and if so, how?"

OK, then please show me how it does NOT "matter".
All available research as I can find is that at .012" or more gap, you could/can end up with a squib. Good question, and THAT is why I asked it.

 

[Sevens]

Oh, I see that despite my best intentions, you were offended.

 

[HorizontalMike]

Oh, I see that despite my best intentions, you were offended.

HUH?

Yes it matters. Why would you think it does not? I apologize, I was looking for data and not opinions. That does make a difference, so if you got confused about offering opinions where data was expected, then I am sorry that you did not understand the difference.

 

[Jeff423]

FWIW: (I hope this isn't too far off topic) Dan Wesson revolvers were supplied with a 0.006" feeler gauge to set the BC gap.
That's where I keep mine. I have tried 0.003" but it fouls too quickly and the cylinder starts to drag.

 

[BLUEDOT37]

I read an article that compared velocity -vs- barrel-cylinder gap several years ago but can't find it right now. As I recall there was less difference than you'd think between a tight gap & a wide gap.

And that test was on the same gun (Dan Wesson, IIRC). You're comparing two different revolvers. It's widely acknowledged that two revolvers that are the same model, cartridge, ammo, & barrel length will not produce the same velocities. (Reference Speer #9: Why Ballisticians Get Gray!)

There maybe 100-200fps difference between two "identical" revolvers. Barrel-cylinder gap is not the only factor that affects muzzle velocity. Even if you knew definitively what difference the B-C gap "could" make, comparing your two guns wouldn't take into account all other factors that could be affecting the MV differences.

I don't think I'd lose any sleep over it until it exhibits other problems like excessive lead/debris spitting.

.

PS: Here's the link to the web page skipnsb mentioned:

BBTI - Ballistics by the Inch :: Cylinder Gap

.

 

[GRT3031]

You know you're getting old when someone refers to a 686 as an "old" revolver.

 

[Hapworth]

The poster from castboolit's approach to measuring barrel-to-cylinder gap is interesting -- somewhat different from what S&W and Kuhnhausen, and has a few odd details.

Empty brass in the revolver for checking carry-up? Yes. For gap? New to me.

Running the action with the feeler in place doesn't allow a free hand to hold the cylinder rearward while measuring -- an integral part of the process.

The before and after proofing, and customer service numbers seem about right; .006" is generally considered the ideal balance between performance and reliability.

Never heard the notion that a .012" gap will stick a bullet in the barrel, and has never personally been experienced shooting that gap and larger.

S&W and Kuhnhausen recommend correcting end shake once it approaches .006".

Never heard of a "long cylinder" approach to correcting gap, though it's interesting and I can see how it might work. S&W pulls the barrel, turns the shoulder back a revolution and refits.

Simply, to measure b-c gap, hold the cylinder rearward and the largest feeler gauge you can comfortably pass between the barrel and cylinder faces is the gap for that charge hole; repeat for each, as sometimes the cylinder face isn't perfectly true (and often the barrel face isn't).

And measuring endshake, the cylinder must be held rearward on the first measurement, but also held forward on the second.

The specs on both your 686s are fine, with the older one slightly preferred.

What really matters is how they shoot...

 

[Hapworth]

...What I am wondering about is... Is ther any way to determine how much energy/fps is lost for each additional .001" of additional gap? I have been searching for this specific parameter, however, I have not found such information thus far...

...Bottom line, I realize that I am going to lose "some" fps with a wider gap, but just how much is the question...

The linked test from Ballistics by the Inch is a good read, and there are a few similar tests others have conducted over the years, but I agree they're few and far between.

The general takeaway is the greatest drop occurs between .000 and .001", numbers we're never operating at anyway, and the remaining drops are lesser and fairly consistent each step of the way out to about .010".

In all it averages to about 10 feet per second for every one thousandth gap increases, though as stated not in equal increments -- the biggest drop is front loaded. I recall that it starts to open up again once you get past .10" or .012", but would have to find the original test producing those results.

For revolvers with varying but in spec gaps, the difference will be negligible compared to other factors affecting muzzle velocity, with BLUEDOT37 spoke well on.

 

[daddio202]

My thoughts on the subject are that if your b/c gap is tad bigger than you would like then just stick to shooting larger or higher weight bullets. I pretty much stick to 158 grain and 146 grain in WC. I never shoot 125 grain bullets in any of my guns as I feel they are not long enough to spread the b/c gap and especially on model 19's create too much forcing cone wear. The heavier weights will keep the forcing cone wear to a minimum. Of course if self defense is the plan then carrying it with 125 grain is not a problem as shooting off only a few in the guns lifetime is going to make no difference at all. I spoke with Hodgdon about using magnum powders and their thoughts agree with this on shooting heavy mag loads. good luck

 

[Hapworth]

This is going to sound flippant and rude, and I swear, that isn't my intention. But... honestly, does it matter?

I believe it would definitely be interesting to know how much difference in velocity you might see but beyond interesting, I am left with "does it matter and if so, how?"

Maybe just ask the question plainly and convey that you're sincerely interested?

One answer is this:

It matters because if you require specific terminal performance from a specific load, and that load has specific (sometimes narrow) parameters (like muzzle velocity) in which it can be expected to perform as designed, you need to know the firearm you're using is capable of delivering the bullet within the critical thresholds.

 

[Forrest r]

If there's a difference of 250fps (That's huge!!!!) between the 2 revolvers, it isn't the cylinder gap.

Cylinder gap ='s +/- 10fps per 1/1000th of gap.

Not only have I read the above statement, I have test it and found it to be true numerous time while working up loads for accuracy in my dw's.

It's common to move the cylinder gap in and out along with raising & lowering the tension on the bbl nut to tighten groups up with the dw's. Changing the cylinder gap has always changed the velocity over the decades and 3 different chronographs. It's always been +/- 10fps per 1/1000th's cylinder gap.

Myself I don't use/shoot anything over 7/1000th's cylinder gap. Start getting over that & things go south real fast. Never worried about squibs, was more concerned with things like flame cutting & forcing cone erosion.

Not a s&w or 586/686 expert by any means, but seeing how you've been so kind to me in the past I'll pass along what I've learned over the years and a little bit of ammo.
Bought my 1st 586 nib back in 1987, darn thing was like a laser.

Back in the 80's surplus powder was everywhere cheap, couple that with cheap primers and free lead. The end result is large amounts of range time. I kept tract of how many primers I bought to make reloads for that 586. At the 100,000 round mark I sent it back to s&w, the timing was wore out. S&W also set the bbl back and redid the forcing cone. At the 200,000 round mark that 586 needed the timing redone again. Got the work done and the forcing cone re-cut. At the 275,000 round mark I noticed the velocities were dropping when using the same loads (100fps drop) I've used/shot/tested in that 586 for decades. I used that 586 as a benchmark when testing firearms of the same calibers. Always take a known performer along with a unknown firearm to the range for the 1st couple range sessions. Anyway that 586's bbl was finely shot out, hence the drop in velocity. Keep in mind that firearm saw it's share of powder puff/target loads. But it also saw countless 1000's of fullhouse 357 loads. Here's what that 586's bbl looked liked when I pulled it.

As the cylinder gap gets larger the flame cutting & forcing cone erosion happens faster. This is why anything over 7/1000th's I don't use/shoot anymore.

It depends on what you want the 686 for. Right now I have 2 of the s&w l-frame series 357 revolvers. The 1st 1 is a 586 l-comp

It is designed for sd plain and simple. It's comped, has recessed cylinders that are cut generously (large at the back end/loads faster & cases fall out quicker) and a 6/1000th's cylinder gap with less than 1/1000th's endshake. Basically it's designed to load/unload fast, not bind & comped for hot loads. The bbl is listed @ 3" but is really 2 1/2" long with a 1/2" comp in the bbl shroud. Even with the 6/1000th cylinder gap & 2 1/2" bbl that 586 has no problem delivering 158gr hp's @ 1200fps on demand with loads that are under max with 2400 & home cast/coated bullets.
Now this 686 on the other hand has extremely tight cylinders. Spent cases from that 586 l-comp need a hammer to get then to seat all the way in the 686. It also has less than 4/1000th's cylinder gap and less than 1/1000th endshake.

Same bullet used in the l-comp 586. But now it's loaded for accuracy in the 686.

2 different l-frame 357's from S&W. 1 is designed/made for sd & the other for target work. One is loose & the other is tight. You need to decide what you want to do with that 686.

IMHO, 8/1000th's is just too much.

 

[HorizontalMike]

Hmm?

forrest r said: "...2 different l-frame 357's from S&W. 1 is designed/made for sd & the other for target work. One is loose & the other is tight. You need to decide what you want to do with that 686..."

Precisely.
What I want to do with this second 686, is to use it as my shooting platform for testing out loads from .38 Special to .357 Magnum. My original 686 is nice enough to NOT want to put it through that kind of abuse as a test-load platform/gun.

I wonder if it would be cheaper/shorter to have a local GS shave 0.005" off of the barrel shoulder, than to send it back to S&W. FWIW, this last trip to S&W took almost 2-1/2 months. Not so much worried about the cost, but more so about the down-time waiting... And final question is, if S&W would actually do the above shaving since they normally consider that 0.010" hold gap to be within "specs"...?

I understand what you are saying about:
"...Cylinder gap ='s +/- 10fps per 1/1000th of gap...." but still wonder out loud if that "constant" may expand exponentially at the greater b/c gaps, especially above 0.008" gap. Surely there must be some point of diminished returns as one approaches the "do NOT fire" gap of 0.012" one would think...

 

[WR Moore]

OK, you can't just shave 0.005" off the barrel shoulder. The front sight would end up at about 9:30 o'clock when properly torqued. You'd have to come back one complete revolution (0.0277 in+/-) and then recut the forcing cone, b/c gap and shorten the extractor rod & center pin.

You bought it as a shooter, why don't you just shoot it as is? Your load to load variations will probably be greater than any variation caused by a larger than ideal b/c gap.

 

[HorizontalMike]

OK, you can't just shave 0.005" off the barrel shoulder. The front sight would end up at about 9:30 o'clock when properly torqued. You'd have to come back one complete revolution (0.0277 in+/-) and then recut the forcing cone, b/c gap and shorten the extractor rod & center pin.

You bought it as a shooter, why don't you just shoot it as is? Your load to load variations will probably be greater than any variation caused by a larger than ideal b/c gap.

I wondered about that, but not having an unmounted barrel to measure the threads, I was/am at a loss. So, the process of shortening the B/C gap is more involved than I had assumed.

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?