CommandCar
.333 member
Reged: 18/09/10
Posts: 292
Loc: East Coast, USA
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In terms of a ball & shot gun, anyone have actual measurements of a 12 bore with invisible rifling (not oval bores)? Land and groove count & width, twist rate, bore diameter, groove depth, etc?
Thanks
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Omnivorous_Bob
.333 member
Reged: 03/10/05
Posts: 286
Loc: Montana
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CC, I can't help you with a 12-bore, but I've got an 8 Jeffery B&S with invisible rifling.
I can feel it, and can measure it with precision tools, and can even see that it is there with a bit of fouling, but would guess the number of grooves as at least 12-15 and have no idea on the twist at all.
My gun was supposedly made for 1250 grain paradox-style conicals, but I honestly can't imagine such VERY shallow rifling imparting any significant spin to a such a heavy bullet. It feels almost like shallow polygonal rifling.
I've of course been wrong before and haven't shot mine past 50, but would love to know if it really had any advantage over a smooth bore. The barrels are a work of art and part of me thinks they never would have gone through such trouble if they really thought it was just advertising hype.
Best Regards,
Bob
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"If we're not supposed to eat animals, how come they're made out of meat?"
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CommandCar
.333 member
Reged: 18/09/10
Posts: 292
Loc: East Coast, USA
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OB,
Have you recovered any slugs, should be able see either engagement or stripping easily.
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DarylS
.700 member
Reged: 10/08/05
Posts: 27490
Loc: Beautiful British Columbia, Ca...
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Bob- with such low speeds, perhaps it doesn't take much to spin them, if groove diameter, and soft. I know soft doens't sound right, but soft gives a HARDER hold on the bore all the way out, with pressure behind continually trying to obturate them.
Many loaders will go harder with shallow rifling - we've had 1 1/2 thou grooves with 18" twist, spin .450" 400gr. paper patched pure lead bullets (in a .458" groove dia.) to 1 1/2" accuracy at 100 meters fired with 100gr. of 2F powder.
For 8 years we tried harder and harder cast and hardened cast bullets tryign to get that Sharps to shoot well. Harder bullets made sense to us since jacketed bullets shot so well in it - then a light came on and we tried undersized, paper patched pure lead - instant success.
Of course, the softer alloys are susceptable to stripping or slugging into cylinders, if driven too hard, pressure wise. There is a ballancing act.
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Daryl
"a gun without hammers is like a Spaniel without ears" King George V
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CommandCar
.333 member
Reged: 18/09/10
Posts: 292
Loc: East Coast, USA
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Bob,
If it's not too much trouble, do you mind trying to count lands and grooves and measure twist on the 8? Also, if it has a 100 yard sight, it probabally works at that range (with the right slug and load....)
Thanks for your imput.
CC
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Omnivorous_Bob
.333 member
Reged: 03/10/05
Posts: 286
Loc: Montana
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CC, I know I have a fairly intact fired roundball from each barrel but I couldn't locate them. The unfired balls were .830" and these each had a small "belt" measuring .8270" and .8265" as near as I could measure. I don't recall any distinct facet or groove markings on them.
Looking over the barrels just now with a bit of lube in the bores to help the grooves (such as they are!)stand out, it looks like they make a 1/3 turn in the not quite 29" inches from the chamber mouth to the muzzle, so I'd call it in the 1:90" +- range. They could easily be 1:100 as they are very faint. I couldn't count the grooves (you can barely feel them) but I'd guess 10-12 of a very shallow polygonal-type rifling.
This gun has fold down sights for 50 and 100 yards. I recall reading that it was meant for 1250gn conicals rather than round balls but don't remember the source. It is slightly undersized for an 8 and was proved at 9/1 despite having the "standard" 3.25" papercase 8-bore sized chamber.
As an aside, I've always found it intersting that, based on Greenhill's formula, the twist required to stabilize a conical drops dramatically as bore size increases. This makes 1:90" twist adequate for a 1.15" long bullet in this case and the 1:100" twist (normal associated with roundball guns) good for 1.5" conicals in 4-bore.
Bob
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"If we're not supposed to eat animals, how come they're made out of meat?"
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CommandCar
.333 member
Reged: 18/09/10
Posts: 292
Loc: East Coast, USA
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Bob,
Thanks for taking another look and posting the info.
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