Optimum Gap between forcing cone and cylinder

[lykoris]

I had read on various places the gap between the forcing cone and the cylinder when the action isn't locked up should be between 2-6 thousands of an inch.

I was checking a 2nd hand 629-1 and the gap seemed extremely tight so I asked the store owner if the gunsmith could measure it - it came back as .02mm or .00078".

So I passed on the revolver. The owner said it was perfectly okay. I don't know a great deal but thought a session at the range with 100-150 44 mag cartridges might be enough to heat the cylinder and lock up the action.

I'm looking for any information that might help to better understand this gap. I don't know if I was right, .00078" seemed extremely tight.

Many thanks.

 

[rcmodel]

I have my doubts the gunsmith had the capability to measure .00078".

Normal micrometers and dial cailpers only measure to .001"
A really good mic can measure to .0001".

And I have never in my life seen a feeler guage or shim stock thinner then .001".

So, I gotta wonder how he came up with a measurement like that?

rc

 

[OldCavSoldier]

I think there was a misplaced decimal point in there somewhere!!

I don't see how somebody can measure 2 hundredths of a millimeter...just doesn't sound right!

 

[Scrapperz]

My 460 XVR is .008" from the factory.

Maybe mines loose....off to the phone to talk to S&W, LOL

 

[rcmodel]

I would bet money the 629-1 was perfectly fine.

If you held it up to the light, you should see just a tiny crack of light.
One Post-It note is about .0035 thick, and if one will fit in the gap, that is enough gap to provide optimal velocity and reliable operation with jacketed bullets..
Two Post-It's is more the norm.
Three or four Post-It's would be getting kind of loose.

rc

 

[lykoris]

Indeed, one decimal place out, I meant 0.2mm

He had a dial caliper in mm, which measured down to tenths of 1mm.

Thanks for the tip RC, I'll use it tomorrow when I go look at a Manurhin.

Cheers.

 

[rcmodel]

Do yourself a favor and pick up a cheap automotive valve tappet feeler guage set. I you buy used (or new anymore it seems) revolvers you need one.

Only $6 bucks at Sears.


rc

 

[Mizar]

How can one accurately measure barrel to cylinder gap only with a caliper? Either the gunsmith is not a "gunsmith" or some info is missing.

Boris

 

[Old Fuff]

A lot depends on what kind of bullets you're shooting. Manufacturers are sometimes going to tighter gaps, in the .003 to .006 range, expecting the user to pick jacketed "high performance" or "tactical" bullets. That's too tight if you use softer lead slugs, and because of that some gaps in older revolvers were as much as .011 - which is a little much, but apparently still acceptable to Ruger. I go with them because I'm more interested in reliability then a few extra FPS, and it's very seldom that I shoot jacketed bullets in a revolver.

 

[dfariswheel]

The barrel/cylinder gap and cylinder end shake are gaged with automotive feeler gages.

First, you should gage for end shake since this affects barrel/cylinder gap.
End shake is the amount of back and forth movement of the closed cylinder in the frame and this affects the actual barrel/cylinder gap.

With the action un-cocked and at rest (don't hold the trigger back and gage, that only gives a false reading), push the cylinder to the rear and hold it there as you gage the barrel/cylinder gap.
Then push the cylinder forward and hold it there while you gage the gap again.
Subtract one measurement from the other and that's how much cylinder end shake you have.

Colt's have a much tighter spec. Anything over 0.003" is out of spec and needs repair.
S&W and Ruger have looser specs, and recently the factories are passing as acceptable more and more end shake.
In general, anything over "about" 0.006" is too much movement.

To gage the barrel/cylinder gap, the cylinder must be pushed to the rear since that's the actual gap.

An ideal gap is "about" 0.005".
Anything under about 0.003" may give binding problems as the gun heats up or the cylinder face fouls.

Again, Colt held the line with their spec being that anything over 0.008" is out of spec and needs repair.
Recently, S&W has been passing gaps of as much as 0.012" as "in spec".

 

[jad0110]

In a defensive handgun, I prefer anything from around .005 to .010. .006 or so is about perfect in my personal book.

If you are going to go .002 (or even .003) or less, the cylinder face must true (most factory cylinder faces aren't perfectly quare) and in perfect alignment in all 3 deminsions with the forcing cone. And "Bogarting" the cylinder (slamming it shut with a Hollywood-style wrist flick) on ANY revolver is a bad idea, but especially on a gun with a B/C gap of .002" or less.

 

[Confederate]

In general, anything over "about" 0.006" is too much movement.

This is mere conjecture and just doesn't jive with the many studies done over the years.

Factory specs for S&W and Ruger are .004-.009-inches. The notion that Colt uses tighter specs means nothing. Based on articles back in the 80s, .006 is the optimum gap size for both reliability and accuracy. While tighter gaps deliver slightly higher velocities, people with Dan Wesson revolvers who set their gaps to tighter settings found that their groups opened slightly.

As B/C gaps are tightened, headspace increases slightly. The key word here is "slightly." The fact is, the S&W/Ruger specs are fine. Headspace gaps from .006-.009 also are optimum, yet I've seen S&W .357s and .44 mags with headspaces of .013, and yet there were no misfires or weak hits. (I've yet to see a Ruger .357 with out-of-spec settings.) I did see a S&W with a B/C gap of .002, and it did bind after repeated firing. Once the gun cooled, it was fine, but this was in the early days of stainless steel.

Dan Wessons come with .006 gauges, and for reliability and accuracy, that size is the way to go. But again, slight variations mean nothing.

As far as end shake goes, when the firing pin hits the primer, it drives the case and cylinder forward. This may mitigate the excess gap size somewhat.

Colt makes fine revolvers, but S&W showed with its first generation of 686s that it could compete with Colt's classic Python (and make it more durable in the process). Colt also never made any attempts to improve its designs. The Python is still plagued with a tiny hand (pawl), which throws it out of time after just a few thousand rounds. Increasing its size or even hard chroming them would have improved the gun, but Colt just isn't into innovation. I wouldn't put too much stock in its specs.

 

[Mizar]

As B/C gaps are tightened, headspace increases slightly.

Confederate, can you explain (and prove) this statement?

As far as end shake goes, when the firing pin hits the primer, it drives the case and cylinder forward. This may mitigate the excess gap size somewhat.

I have yet to see a main spring capable of withstand the recoil's force.

P.S. I believe that Dfariswheel knows verry, verry well what he is talking about.

Boris

 

[dfariswheel]

In general, anything over "about" 0.006" is too much movement.

This is mere conjecture and just doesn't jive with the many studies done over the years.

Factory specs for S&W and Ruger are .004-.009-inches. The notion that Colt uses tighter specs means nothing. Based on articles back in the 80s, .006 is the optimum gap size for both reliability and accuracy. While tighter gaps deliver slightly higher velocities, people with Dan Wesson revolvers who set their gaps to tighter settings found that their groups opened slightly.

0.006" is not barrel/cylinder GAP, that's END SHAKE, the amount the cylinder is free to move back and forth in the frame.

As I said, S&W has been passing barrel/cylinder GAP of as much as 0.012" as "in spec".
However, if your cylinder is moving back and forth more than 0.006" the gun needs repair before it literally hammers itself to death.

 

[Confederate]

Actually, I misunderstood his measurement (mixing it up with jad0110's post) and don't have a huge problem with it. I've seen more endshake in revolvers than what he mentions and they function fine. And for the price Colt wants for their guns, the specs should be tight.

When the hammer hits, things happen so fast that we sometimes don't have a good idea of what happens before the bullet leaves the barrel. Back in the 80s, when people were really studying these effects, it was learned that some of these specs weren't as meaningful as we thought. But one of the biggest problems with endshake is that play begets more play. Recoil tends to batter guns and open gaps if they're not designed well.

Ruger tend to build its guns to resist recoil -- blasts, gas and warping, and specs having to do with barrel diameter, cylinder throats and alignment have far more to do with accuracy than gaps. That's not to say gaps aren't important, but if someone has a headspce gap of .013, when the specs top out at .009, and yet the gun never misfires -- and the groups are great -- is it worth sending back to the factory?

Years ago I had a Taurus with a headspace gap of .013 and a B/C gap of .006. It worked fine, but I returned it. When I got it back, the headspace gap was .009, but my B/C gap was .010. The gun didn't misfire before so what did I gain? So I got rid of the gun. It was horribly inaccurate, but this was largely because the cylinder throat size was almost nonexistent! Even .358 SWCs would drop right through the chambers (this didn't happen with my Rugers and Smiths) -- so now I don't buy Tauruses. I had several bad experiences with them, and there were many tolerances out of spec. It was almost as if everything was going into the spit and polish. The guns were outwardly gorgeous, but accuracy was atrocious. (Things may have changed since then -- this was back in the 80s.)

 

[SWAddict]

I have 13 S&W revolvers, some used and some new. They have gaps ranging from .002 to .010 and they are all accurate and work perfectly. They do not spit or shave lead.

I would say that barrel to cylinder gap is not important as long as you are not on the extremes; not less than .002 and not more than .012 or so. A larger gap will decrease velocity slightly but not enough to notice or matter.

What is more important is timing and cylinder bore to barrel alignment. If you are good on this then you are good to go.

If your barrel to cylinder gap is large but your revolver shoots and functions well, don't worry about it.

 

[lykoris]

Thank you for the useful points of information dfariswheel.

 

[Confederate]

...S&W has been passing barrel/cylinder GAP of as much as 0.012" as "in spec" ... if your cylinder is moving back and forth more than 0.006" the gun needs repair before it literally hammers itself to death.

Although I initially misunderstood your figures as far as gap sizes go, the endshake issue also is one we need to fully understand.

How much extra money are we willing to spend to keep these specs at reasonable levels? And should the specs be the same for S&W, Ruger and Colt?

Despite Colt's .0006-inch spec, can we assume that Colt revolvers are more [i[]robust[/I] than Ruger's or S&W's if those revolvers are set at .0012? Or are they more accurate?

I'm no gunsmith, but I do know that if I were going to be dumped in the remotest part of the American Northwest after a global societal meltdown, and if ammo wasn't a problem, but spare parts and repairs were, I don't know too many people who would take a Colt Python or Trooper over a S&W 686 or a Ruger GP-100. Nor would most people want to pay $250 more for these guns just to have the tighter tolerances. In short, if any of these guns would get "hammered" by the repeated firing of hot magnum .357 loads, I think Rugers would take first place, S&W second (K-frames excluded), and Colt last. The newer designs of the S&W L-frames and Ruger's "Six" .357s and GP-100s allow them to take more brutal treatment and be produced more inexpensively and effectively. Ruger's investment casting and innovative solid-frame design are particularly effective in increasing strength while keeping costs down.

As far as end-shake is concerned, I'm willing to trust S&W/Ruger's specs to keep costs down. Excess end-shake should be something one can feel without a gague, just as excess B/C gap can be detected by people who are used to measuring B/C gaps.