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Is there any rule of thumb regarding the increase in velocity as the barrel/cylinder gap is reduced? I would imagine that the velocity increase per 0.001” bc reduction would vary depending on whether it was from 0.008” to 0.007” or from 0.003” to 0.002” since the reduction from 0.003” to 0.002” is a greater percentage reduction than that same 0.001” increment at the larger bc gap. I have a Ruger Bisley 45 Colt and it has 0.008” gap and I am getting 1200 fps from a hard cast 335 grain bullet. What would the velocity likely be if the bc gap was reduced to 0.003”? Thanks, Brian.
 

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The Hog Whisperer (Administrator)
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Brian,

Interesting question. I would be fascinated to know the answer myself.

Actually, another way of stating it would be, how much does the max pressure of the load change if the B/C gap changes?

The B/C gap, along with the inherent freebore (distance before the bullet hits the rifling) in a revolver reduce both pressure & velocity somewhat.

So..... if a person were to send a gun to a smith & get the B/C gap reduced 0.005", would there be a significant jump in pressure, and would loads have to be re-worked to avoid excess pressure?

By the way... you did not ask... BUT ... you might be interested to know that a 335gr. .45 cal bullet will go through a wild hog lengthwise, at less velocity than what you are getting. So, that large B/C gap that you mention isn't necessarily costing you a lot of performance, unless you shoot silhouette, and are tired of catching your pants leg on fire.....

If you find the answer somewhere else, be sure to let us know. Like I said I'd be real curious to see if anyone has done experiments to put some numbers to this question.
 

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Linebaugh says

John Linebaugh said at the seminar that a barrel cylinder gap of .002 would produce about 20 fps less than a pressure barrel.
 

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I saw the same question posted on John Taffin's web site. Thought you might be interested in the answer that was posted by PACO:

"PACO
Fri Aug 16 20:48:09 2002
65.103.210.233
A LOT HAS TO DO WITH THE TIGHTNESS OF THE BORE ALSO
WE DID TESTS WITH DAN WESSON 357s...YOU ARE RIGHT ABOUT IT BEING DIFFERENT FROM 8 THOU DOWN TO ZERO....BUT FROM 4 THOU DOWN 1OOOTHS AT A TIME IT AVERAGED 100 FPS WITH A 180 GRAIN BULLETS (JACKETED) AND 13.5 GRAINS OF 2400..STANDARD PRIMERS"

God bless
 

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"Bad Joke Friday" Dan (moderator emeritus)
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Not too tight

I don't know the impact for velocity or pressure, but I do know you can get the gap too tight. I've had several revolvers that were custom reworks that I had to send back because the gap was too small and after shooting several loaded cylinders, the cylinder would require manual help to rotate because of the residue/contamination in the small gap.


Dan
 

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Brian,

I began with a bone-stock (except for fire-lapping the barrel to remove the barrel-shank constriction) Ruger Bisley Vaquero, 5.5" in .44 Magnum, the gun measured .0085" for cylinder/barrel gap. I worked up a load (not max, but very accurate) of .432"-280g WFNGC/22.5g H110/WLPP/Starline Brass for a velocity of 1288 fps.

I sent the gun to David Clements for some work, and among the things he did was to bush the cylinder to remove excessive endshake, and set the barrel back for minimum cylinder/barrel gap. The gap measured .0015" when it returned home to me. Then shooting some of the ammo (as per recipe above) I had already on hand loaded up from my last chronograph session, I got 1397 fps average for 12 shots fired over the skyscreens! The only change in the gun was basically the cylinder gap.

I don't know if this is representative of typical results or not, as I've not done in-depth testing of various cylinder/barrel gaps, but it would make an interesting study to be sure.

FWIW,

God Bless,

Marshall
 
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