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More On Flash-gap Velocity Loss
#1
More on Flash-Gap Velocity Loss

Posted By: ML - Registered User

Posts: 454

Posted At: (8/25/03 3:04 pm)

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More on Flash-Gap Velocity Loss



A while back, (last October to be specific), I posted some data in this Forum concerning barrel length and its influence on velocity, specifically in regards to handguns. In the subsequent thread, I also discussed the influences of a revolver’s flash gap (the distance between the front of the cylinder and the forcing cone of a revolver’s barrel).



Since then, I have uncovered some additional information.



One tester’s results have shown a drop of approximately 15 fps (feet per second) in muzzle velocity for every 0.001-inch of flash gap. The tester suspects this may be slightly higher for hotter, high-pressure handgun cartridges such as the.480 Ruger or the .454 Casull, although he did not test for this.



A revolver must have a flash gap in order to function correctly (the M1895 Nagant the notable exception). If the gap is too tight, reliability suffers. The SAAMI lists 0.012-inch as an industry maximum for revolver flash gap. Most current revolvers from our better manufacturers (Ruger, Smith and Wesson) beat that substantially in their tolerances. Personally, I feel a flash gap of between 0.003- and 0.004-inch is about ideal, and even 0.006-inch is nothing to raise an eyebrow over.



For the mathematically challenged or heinously lazy, a gap of 0.012-inch should result in a theoretic loss of 180 feet per second, while our ideal range of 0.003- to 0.004-inch results in a loss of just 45 to 60 fps—about four percent for a .357 Magnum-class cartridge.



Should you be interested in measuring your revolver’s flash gap, all you need to do is insert a common automotive feeler gauge such as used to adjust valve clearances between the cylinder’s face and the revolver’s forcing-cone area. Please make sure the firearm is unloaded first, and measure the clearance with the cylinder already in the latched-shut position (that is, do not attempt to close the cylinder on the gauge’s blade).



I’m not sure I’d view this information as gospel, since it’s unclear under what atmospheric conditions these tests took place, and how strict the controls were. Still, though, I have enough faith in it as a ballpark figure to share it with you here.



--ML





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Re: More on Flash-Gap Velocity Loss

Posted By: wmerrin - I survived WASP

Posts: 915

Posted At: (8/25/03 6:36 pm)

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ML's clearance numbers are of some significance. Years ago my brother bought a Ruger Stainless Security Six in .357. After firing several rounds through it the action would start to drag, and after one or two more rounds it would sieze. We determined the gap was too tight by comparing it to an identical Ruger of mine. Expansion from the heat of firing would make the gap go away. Because of that experience, not much later I was able to diagnose another guy's problem at a range when he was saying unkind things about a Ruger he had.



Wally

==================

Of all the things I've lost, I think I miss my mind the most.

Wally Merrin

wmerrin@earthlink.net



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Re: More on Flash-Gap Velocity Loss

Posted By: Birdog - Registered User

Posts: 3164

Posted At: (8/26/03 4:58 am)

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Wally,



I think the more likely culprit would be powder residue, and not heat build up, although that would also be a contributing factor. If you had removed the cylinder and cleaned the face, I'm sure it would have solved the problem, at least until you fired a few more rounds.



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Re: More on Flash-Gap Velocity Loss

Posted By: wmerrin - I survived WASP

Posts: 918

Posted At: (8/26/03 1:12 pm)

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No, it was a manufacturing error. The clearance was way too tight. I measured with feeler gauges and compared with a known-good model and they were significantly different. I solved the problem on my brother's by very carefully removing metal, with frequent stops to measure. This was years ago so I don't remember what the numbers were before or after, but the problem went away when I was done. In retrospect, the gun should have been returned to Ruger since the problem occured out of the box, but my brother didn't want to send it back.



Wally

==================

Of all the things I've lost, I think I miss my mind the most.

Wally Merrin

wmerrin@earthlink.net



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Re: More on Flash-Gap Velocity Loss

Posted By: Birdog - Registered User

Posts: 3173

Posted At: (8/27/03 5:28 am)

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>>No, it was a manufacturing error. <<





I agree! I was just stating the reason why it froze up after a few shots.



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Just to add a little information

Posted By: Handy - Registered User

Posts: 118

Posted At: (8/26/03 4:58 am)

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When checking clearence with the feeler gauge insert the guage after the the guns trigger and hammer are in the fired position and the cylinder is locked in position. I like between .004" and .005" flash gap. insures good function and probably better accuracy.
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"In the school of the woods there is no graduation day" Horace Kephart


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