The REAL Effect of Barrel/Cylinder Gap

[Confederate]

If you thought this was going to be an answer to the question, no such luck. I just wanted to toss it our for discussion. In the 70s, more was written about the effects of barrel/cylinder gaps, not only for velocity, but accuracy.

The specs for revolvers in those days were simple: acceptable B/C gaps were anywhere from .004 to .009 inches. Some gun writers back then attempted to find various models of the same gun with different gaps and compare them, but individual differences in the guns themselves interfered. The barrel specs on two guns of the same make and model could easily account for differences in velocity and accuracy. Some of the more enterprising gun writers used Dan Wesson revolvers with gaps set to different sizes. (DW revolvers came with a gauge to set their guns for .006, which was the industry standard goal.)

Back then, .357 mags were the hot setup, so most writers used them, though some tried .22LR since DW made revolvers in that caliber as well. Many buyers back then wanted to milk as much velocity out of their .357s as possible and the big myth in those days were that a .357 really didn't produce magnum velocities in guns with barrel lengths under 6-inches. So they wanted to find guns with .004 gaps. Some writers, however, said that while velocity increased slightly with tighter gaps, accuracy tended to be best from guns with .006 gaps.

Now some people think they got burned if their B/C gaps are .006 or .007 (and some will go so far as to want to return guns with .007 and over), but I haven't seen anyone actually do any studies on B/C gaps recently. I also don't know if revolver manufacturers have changed their specifications since the 70s. I just picked up a .22LR with a .010 gap because it seems to shoot very well, doesn't spit and is as accurate as my others. It was a good price and I couldn't pass it up.

But most of what's on the Internet is opinions. No one really has actually compared guns to see what effect the gaps have. One source even stated that 30 percent of the blast could be wasted through the B/C gap. That doesn't seem right based on what I've read. Back in the 70s and early 80s, gun writers tended to be more oriented towards things like this. Now most writers just want to fill up space describing guns that are already photographed in great detail and spout unfounded opinions.

What do you think? Has anyone read any articles backed up with empirical data? Or has anyone done any studies on this themselves?




This is a revolver held up to a strong light. The gap is not actually as
pronounced as how it appears here.

 

[RevolvingGarbage]

Heh, the Brl/cyl gap on my RG-40 is actually about what it looks like that one is. It holed an old junker fishing boat pretty good with only target FMJ so im gonna say the velocity loss is acceptable, if not negligible .

 

[Hawk]

Has anyone read any articles backed up with empirical data?

Yup. But it's not very extensive and conclusions are elusive. Better than straight opinion though.

http://findarticles.com/p/articles/mi_m0BQY/is_1_51/ai_n7581216/

This little snippit is intriguing but may require research involving dead trees and pigment rather than intertubz browsers.
http://forums.1911forum.com/showpost.php?p=1603936&postcount=13

 

[MCgunner]

Don't you have a couple of 511s with normal gaps? Simple test, just shoot 'em over a chronograph. Don't have one? They're under 100 bucks for a Shooting Chony. I gave 60 bucks for mine 25 years ago, so that's still a pretty good bargain.

 

[woad_yurt]

I have an old, very small .22LR revolver made by the US Revolver Co (Iver Johnson.) It has the biggest barrel gap I've ever seen. I also have an H&R snubbie with the same sized barrel and a tiny gap. They both penetrated pretty much the same when I shot some 2X4s from a foot away.

 

[harmon rabb]

screw the gap. get a nagant 1895

 

[Shadan7]

We're currently getting together the ammunition for testing in April:

Announcing the Cylinder Gap Test!

We’ve had a Single Action Army clone in .357 magnum modified to allow for adjusting the barrel position from a standard 0.006″ to 0.001″ to no gap (barrel snug against the cylinder). We have a dozen or so different ammunition loads in .38/.357, and we’ll be essentially repeating the BBTI procedure for each of these, with the normal gap then the minimum gap then without any gap, starting with an 18″ barrel and going down in increments of one inch to just 1″ . Actually, one slight difference – to make sure we get a better statistical sample, we’ll be firing 10 (ten) rounds of each type of ammunition at each point rather than just 3 (three) as we did with the BBTI tests. Because we are limiting this test to just one caliber, we thought this was a reasonable step to take. We hope that this will allow us to conclude with some actual data what the effect of having a cylinder gap in a revolver actually amounts to.

It total, it amounts to about 7,000 rounds to be fired. Hope to have the results available by summer.

Jim D.

 

[1911Tuner]

The difference in velocity between .004 inch and .008 inch is so insignifigant that it can be ignored. To apply it in choosing two identical revolvers in the belief that the one with the smaller gap will produce a higher velocity at all...let alone one that actually means anything...is to consider only one aspect of it. There are too many variables that affect velocity between identical guns to worry over a gap difference that's less than the thickness of typing paper.

The factor that has a much greater effect is the forcing cone itself...or specifically...the angle of the cone.

Ruger barrels have a reputation for "shooting fast" with a given barrel length. Compare the geometry of the forcing cones between Ruger and Smith & Wesson for the answer.
Ruger forcing cones are nominally cut at 5 degrees. Smith at 8 degrees. That provides a larger expansion chamber for the gasses as the bullet passes through the cone and into the rifled portion of the barrel. The gasses and pressure fill the cone more readily than they can escape through a tiny .006 inch gap.

Many things affect the muzzle velocity. The barrel cylinder gap is only one...and it's not a very signifigant one at that.

 

[nitetrane98]

I don't know how you could go about doing such a test that would give empirical evidence of the effect of minute cylinder gap differences. First and foremost would be to somehow make ammo that was precisely the same velocity from one shot to the next. Anybody with a decent chronograph will tell you that won't happen. At best I think you might be able to assign a percentage of gain or loss based on gap. It seems obvious that the smaller the gap the more velocity. Testing would require that the barrels and cylinders be the exact same temperature for each shot, the exact same cleanliness. Then you can go ahead and try to factor in barrel wear. It's there, could it be measured?
Everything, it seems, is a compromise with guns. There is a price to be paid for a .002 gap. Like not being able to fire a box of shells through it without having clean the front of the cylinder to keep the gun from binding up.

I think there is a good reason for the dearth of empirical data on the subject. The knowledge that could be gained is not worth the effort.

 

[MCgunner]

Ruger barrels have a reputation for "shooting fast" with a given barrel length. Compare the geometry of the forcing cones between Ruger and Smith & Wesson for the answer.

Man, I learn something every time I boot up this site. Thanks. I've always heard that and I knew it to be a fact with my old Security Six, but I never got a good explanation as to WHY. Now, I have more knowledge.

 

[Confederate]

Comparing the 511 with the .010 gap with my others most likely wouldn't render suitable results because each gun has different specs in barrel, throat and other dimensions.

People who advocate negligible differences in gap sizes note that the bullet already has left the gun by the time most of the escaping gas is getting around to leaving the gap area. I've seen photos of people shooting their guns where the gas is just leaving the barrel, but is mostly just flashing the gap area. In other words, most of the escaping gas doesn't do so until the bullet has left the barrel.

 

[Confederate]

The difference in velocity between .004 inch and .008 inch is so insignifigant that it can be ignored. To apply it in choosing two identical revolvers in the belief that the one with the smaller gap will produce a higher velocity at all...let alone one that actually means anything...is to consider only one aspect of it. ...

Yes. There are many variations between two guns of the same model as to queer any comparisons. With ammo variations, you can do standard deviations, but there are too many other variables, as you noted.

Again, Dan Wessons might be the way to go, with standard deviation charts.

I recall in some of the 1970s articles that some guns with 4-inch barrels exceeded guns with barrel lengths of 6-inches in velocity. Some chalked it up to barrel/cylinder gaps, but it didn't pan out.

 

[whistler]

would not the variations in each chamber of the cylinder cause as much if not more a difference than the b/c gap

 

[rcmodel]

There was a very comprehensive test done by Edward C. Yard (A very respected Ballistician of the time) in the Gun Digest Treasury 1969 edition.

Yard used a modified Colt Single Action with a barrel that could be adjusted for gap.
He used a pendulum type free-swinging front sight to maintain a front sight picture at any barrel setting.

Bottom line.
.22 Hornet:
.000" = 1655.
.001" = 1595
.002" = 1585
.003" = 1565
.004" = 1545
.005" = 1540.
.006" = 1520
.008" = 1485
.010" = 1435 - stopped

Testing in .22 Hornet was stopped at .010" due to excess cylinder end play at that loose barrel setting with .22 Hornet pressure slamming the cylinder back against the firing pin.
It was causing the Colt SAA to re-cock itself and attempt to go full auto!!!

With .38 Special barrel in place:
.000" = 985
.001" = 965
.002" = 955
.004" = 935
.006" = 920
.008" = 905
.010" = 890
.012" = 865
.015" = 835
.020" = 795

So there it is!
The first .001" of gap causes the largest velocity drop of all.

rc

 

[Bailey Boat]

May not be relevant in this discussion but more of an observation from personal testing.

I was working up a load to be used in a mod 19 (.06 gap and 4" bbl) and a mod 60 (.03 gap and a 3" bbl) and consistantly (85% of the time) recorded higher velocities (using a chrono) from the 3" barrel. Both guns are chambered for .357 Mag and the chrono was never moved during the testing.

I don't know what to make of it since I should be getting lower velocity from the 3" gun than from the 4" but those were my observations during the course of testing about 6 or 8 different loads using 5 different powders.

 

[Confederate]

Thanks, rcmodel! This is exactly what I was looking for, and the date is significant -- they just don't do studies like that any more.

The factory specs back then were .004-.009, and Dan Wesson supplied a .006 gauge to set their revolvers. As stated earlier, .006 was the target. Most of my Ruger .357s are .004-.005.

The loads listed, in specifications, are:

.22 Hornet

.004" = 1545
.005" = 1540
.006" = 1520
.008" = 1485
.010" = 1435

.38 Special

.004" = 935
.006" = 920
.008" = 905
.010" = 890

In spec, the loss is minimal in both cases. The .22 Hornet is high pressure while the .38 Special is relatively low pressure.

Thus, the question seems to have been answered.

 

[1911Tuner]

Bailey...It's not unheard of to see more velocity from a shorter barrel. Not common, but not all that rare, either.

I recall in some of the 1970s articles that some guns with 4-inch barrels exceeded guns with barrel lengths of 6-inches in velocity. Some chalked it up to barrel/cylinder gaps, but it didn't pan out.
__________________

Confederate...So many variables, so little time and money to do definitive testing.

A few years back, a gun writer did an article on rifles and velocity gain/loss with different barrel lengths. He did several test lots, and recorded the results along with the handload data...then unscrewed the barrel...lopped an inch off, and recrowned...and did it all again. Seems like it was a .308 caliber 700 Remington...but don't hold me to it.

With a couple of the faster powders...3031 and 4895 I think...he actually got more velocity with 2 inches less barrel length. One was quite a bit more.

So, toss in another variation. Powder burn rates.

 

[The Lone Haranguer]

I don't know how you could go about doing such a test that would give empirical evidence of the effect of minute cylinder gap differences.

You could use a Dan Wesson revolver. Start out with, say, a .010" gap, then progressively reduce the gap .001" at a time.

 

[bluetopper]

I get more velocity out of a 6.5" revolver barrel than I do a 5" 1911 shooting 200gr cast lead SWC 45acp.

 

[The Lone Haranguer]

I still get more velocity out of a 6" revolver barrel than I do a 5" 1911.

In this case (cylinder gap notwithstanding), the entire six inches (minus the forcing cone) of the revolver barrel is actual rifled bore. The five-inch auto barrel includes the chamber, so its actual length of rifled bore is ~4 inches.

 

[1911Tuner]

The five-inch auto barrel includes the chamber, so its actual length of rifled bore is ~4 inches

4.1 inches...but ain't nobody pickin' nits here.

Wait! Hold that. A 1911's nominal barrel length is 5.02 inches..so that'd be 4.12 for a 1911 if it's to spec.

I know. I know.

*groan*

 

[bluetopper]

I used to think you would have some noticable power loss from the B/C gap but after some experimenting, the power loss is very negligible in my opinion. I'll see if I can find the numbers I chrono'd about three years ago and get back with yall.

 

[unspellable]

I've done this test using Dan Wessons chambered for 22 LR and 44 Magnum. Fired enough rounds to begin to be statistically significant and kept track of chamber to chamber variations as well. Changing the gap did not affect velocity in ways you would expect, nor did it change velocity by all that much. Unfortunately, I've recently moved and the data is buried someplace.

By the time the cylinder gap reached 0.010 or 0.012 inch the revolver began to spit crap back in my face, so that's one argument against excessive gap.

For accuracy, Dan Wesson ran some tests that the other makers should have done 70 years ago. The finding was that the relationship of bullet size, throat size, and groove diameter were the three most impotant factors. Twist rate counts. (DW was the only manufacturer to offer more than one twist rate.) Muzzle crown job counts. Cylinder gap was pretty low on the list so long as it was consistent from chamber to chamber.

 

[James T Thomas]

gap vs no gap

Perhaps fire a compatable pistol-revolver round in equal length barrels of both the revolver and semi automatic.

For example: the 9 mm round.
Shoot and chrono it from several revolvers; like those with the interchangeable cylinders and then select several pistols with the same, or even close to the same, barrel lengths.

You might get information if you had several revolvers with different gaps and compare those to the pistol data.

What others? 45 ACP, ...

 

[DWFan]

The difference in my 10" Dan Wesson .357 Maximum SuperMag from the factory delivered .006" to .002" was approx. 70 fps. I say approx. because with some powders the velocity deviation proved to be greater than 70 fps.