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Thursday, 31 March 2016

Cylinder Gap Power Loss and Gun Digests:

That hot gas leaking from the cylinders of revolvers does cause a loss of velocity - as has been shown in tests by

- But there has to be a gap there to permit the cylinder to rotate. - Too little and you have a revolver that tightens-up and jams - too much and your wheel-gun may dangerously spit lead and hot gasses at anyone close by - including you the shooter. - Shouldn't you be wearing safety glasses?

Way back in near prehistoric times - a learned gentleman Edward M Yard built test guns with the cylinder gap adjustable from zero upwards using a micrometre scale to change the gap by rotating the barrels and then firing them through a chronograph.
Scale Marked in Thousandths of an Inch.

Mr Yard tested his gas seal at a zero-gap by wrapping white paper around the cylinder and by seeing that there was minimal staining and no damage - whereas even a 0.001" gap destroyed the paper.
(- "Hot" high velocity ammo is well known to cause flame-cutting to revolver top straps and it erodes forcing cones).

He published his findings in the 1964 'Gun Digest' in a story titled "Gas Loss In Revolvers" and I show some extracts below .. Basically both sets of tests demonstrate a measurable drop-off in velocity as the cylinder gap increases - but differences in the cartridge loads were found to be equally significant and great effort needed to be made to load accurately and to eliminate that variable.

Author Edw. Yard did a huge amount of research and precise re-loading for his small three page article - even building non-fixed front sights suspended from the muzzle - that stayed vertical as the barrels were rotated by using a pendulum effect.

The modern Gun Digest continues to be a good read - but they don't seem to publish 'in depth' technical stuff any more - must be something to do with the happening "information age" eh - and the print size has got much larger - so you get less words for your money!

Currently it seems usual to have from 0.003" up to 0.008" revolver cylinder gaps - while other variables like the barrel 'forcing-cone' & bore and groove dimensions can also have marked effect on performance..

You can conclude that we need a minimum gap that permits the revolver to function - but I also found online that the cylinder gap and end-play slop may be corrected by use of shims.

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

- You might say that this roughly averages as a loss of 10 foot per second velocity for every one thousandth of an inch extra cylinder gap..

Mr Yard did more tests using  .22 Hornet Calibre barrels - believing that the higher velocity round might show greater variances - but he had to stop the testing early when the cylinder started to bounce with the wider gaps and caused it to dangerously rotate and even re-cock the hammer when fired.

- The only well known revolver to address this issue of propellant gas loss at the cylinder-barrel gap is of course the M1895 Nagant with its reciprocating / rotating cylinder. Nobody else seems to think that it is worth worrying about eh - But Nagant experts claim that the gas seal yields some extra 50 -150 ft. per sec. velocity at the muzzle when compared with normal leaky structures and that appears to be about right judging from the experimental results.

Colt FitzGerald Special.

Note: I've also been reading 'Shooting' published 1930 by J Henry Fitzgerald (well known for his "Fitz Special" revolvers) - and in his Q&A section he writes:

"Q. Does clearance between cylinder and barrel affect the accuracy?
A. Yes, if it is in excess of the normal distance, three to five thousandths, the clearance must be greater in the .22 calibre revolvers than in the larger calibers due to Lesmok powder ('less smoke' -get it?), which will bank up on the end of cylinder and barrel causing the arm to jam if it is not wiped off after every forty or fifty shots. There is no advantage in a distance under three thousandths between cylinder and barrel because, due to collection of powder residue on end of cylinder and barrel, one half of that distance will be closed."

"Lesmok" (Winchester) powder was developed in 1911 by DuPont and widely used in .22" rim fire cartridges. It was a blend of black-powder and nitrocellulose that was less smokey and less corrosive than straight black-powder.(But still smokey and corrosive!)

Marty K.