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Old July 9, 2014, 10:49 AM   #1
publius
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Cylinder gap pressure

How much energy is lost at the barrel/cylinder gap. I've never seen it talked about but the gap has to affect pressure and velocity?
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Old July 9, 2014, 11:25 AM   #2
MrBorland
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AFAIK, it has very little effect on muzzle velocity.
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Old July 9, 2014, 12:59 PM   #3
.22lr
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http://ballisticsbytheinch.com/gaptests.html

There isn't enough difference to matter.
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Old July 9, 2014, 12:59 PM   #4
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It depends a lot on a lot of things, but I have seen 50 fps loss stated with .006-007" gap. That is not insignificant, but not really a lot in an adequate load.

Most tests are x gap vs. no gap, but "no gap" in a revolver is impossible*, and any gap under .003"-004" can result in cylinder binding when the gun heats up from firing.

*OK, OK, I know about the Russian Nagant, so anyone who carries one can get a bit better performance. I also know about the Savage percussion revolver; anyone who carries one of those is lopsided.

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Old July 9, 2014, 01:05 PM   #5
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I think it's relative to the amount of gap, & the starting pressure of the cartridge... would make sense that a 460 S&W, would lose more pressure at the gap, that a 45 Auto... of course the 460 starts with more pressure...

I know I've been inadvertently burned by higher pressure cartridges, & not ever been burned by lower pressure cartridges
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Old July 9, 2014, 02:38 PM   #6
Sevens
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I have chrono'd the same exact handloads through a 6" revolver and a 5" Coonan Classic and always (always always always) get 50-80 fps more from the shorter (but sealed) semi-auto Coonan.

A fine way to actually test the loss from a flash gap would be with a Dan Wesson revolver. It's extremely easy to set any range of flash gap you like with one of these guns. When you set the gap too short, the revolver heats up, expands, and ends up binding, but that takes a few shots before it happens.

Your test results would not be definitive and/or the final word, they'd simply be a window in to the whole discussion... as every different handgun with differing chambers / barrels / etc will perform a bit differently, but it would be better method of testing than comparing two different handguns.
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Old July 9, 2014, 08:09 PM   #7
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Nothing to worry about imo
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Old July 9, 2014, 08:26 PM   #8
gyvel
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*OK, OK, I know about the Russian Nagant, so anyone who carries one can get a bit better performance.
There were contemporary revolvers made for the Nagant round (mostly, if not all, Belgian knock offs) that did not have the moving cylinder feature. It's been a lot of years since I have read the results of gas seal vs. non gas seal, but I believe the difference was slightly less than 100 fps.
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Old July 9, 2014, 09:01 PM   #9
Jim Watson
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The tests have been run... if you have the library for it. Unfortunately, I no longer do.

But I recall trials with a Dan Wesson, and even before, somebody screwed a barrel in hand tight on a SAA and made a pendulum type sight for it so it could be aimed across the chronograph. Then there was the setup made to test an actual Nagant setup in a gun that could be fired gas sealed and not.

Speer made a one shot "cylinder" for a revolver with a pressure transducer. They found that maximum pressure was reached just about as the bullet was bridging the gap.

I think the FA guns have about a .002" gap to get the most out of .454 and overloaded standard calibers. Good alignment avoids dragging, hot loads burn clean and reduce fouling.
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Old July 14, 2014, 10:12 PM   #10
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Just wondering what everyone's opinion was. It's always puzzled me how a longer barreled revolver can have higher muzzle velocities than a shorter, all else being equal. It would seen that the cylinder gap would be sufficient to relieve the pressure and cease or diminish acceleration down the barrel.
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Old July 19, 2014, 10:38 PM   #11
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There was a revolver whose cylinder was designed to close its gap completely upon firing, and then upon rotation, back off of its cone and then rotate to its next hole. I think it was Russian, designed for suppressor. The muzzle velocity difference showed to be like 30 fps or something. I believe it was determined to be too much complication involved for such a small gain.
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Old July 19, 2014, 11:00 PM   #12
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Quote:
Just wondering what everyone's opinion was. It's always puzzled me how a longer barreled revolver can have higher muzzle velocities than a shorter, all else being equal. It would seen that the cylinder gap would be sufficient to relieve the pressure and cease or diminish acceleration down the barrel.
You can only get so much gas through a small gap in the time it takes to get the bullet down the barrel.

Think about a tire with a leak in it. It doesn't usually go flat all at once. If the hole is small enough, it may actually maintain usable pressure for days. A larger hole may bleed pressure off in just a few minutes, but it still takes some amount of time.

The bullet is in the barrel for a VERY short amount of time, and while there's a leak at the cylinder gap, it's a relatively slow leak. So in the same way you can air up a tire and there may be enough air in the tire make it home even though it's leaking, there's still enough usable pressure in the barrel to accelerate the bullet even though it's leaking out around the cylinder gap.
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Old July 20, 2014, 09:19 AM   #13
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Originally Posted by Crankgrinder
There was a revolver whose cylinder was designed to close its gap completely upon firing, and then upon rotation, back off of its cone and then rotate to its next hole. I think it was Russian, ...
Yes, the Nagant revolver, which has already been mentioned in this discussion.
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Old July 21, 2014, 11:53 PM   #14
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I have a 7.62 nagant revolver, and learned to hand load for it. I tried two different approaches. One with short brass, so that there is effectively no gas seal. The other was with long brass so that the gas seal was intact. Exactly the same of everything else, the speed difference was 10%, or 60 to 70fps. That is 20% difference in energy.

I would say the difference is significant.

Having said that, this is mostly academic. Most revolvers require cylinder gap to function reliably. There is reason the nagant became obsolete.

-TL
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Old July 22, 2014, 12:10 AM   #15
Sevens
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Most would argue that it was born obsolete, or worse.
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Old July 22, 2014, 12:26 AM   #16
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Obsolete nonetheless.

-TL
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Old July 24, 2014, 02:27 PM   #17
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It would be interesting to take short, high pressure cartridge like 9mm and chamber it in a relatively long cylinder to see what effect moving the gap away from peak pressure has.

In other words, if the pressure loss at the gap was relatively low pressure, would the overall effect on velocity be the same as a gap that is closer to the cartridge and at higher pressure?

I suppose a comparison could be done with a .357 and 9mm loaded for a Ruger convertible Blackhawk, but you'd need two Contender barrels to independently match the handloads.
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