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Old 10-26-2020, 12:21 PM
mscampbell2734 mscampbell2734 is offline
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Default Explain why cylinder throat diameter

Didn't want to hijack a thread so thought I'd start a new one.

I know if the throat diameter is SMALLER then bore you'll have issues no matter what. The bullets gets swagged down and is undersized to the rifling.

BUT my question is why a oversized throat, in this case we're talking 9mm revolver at .360, would cause issues with cast bullets, but not really affect jacketed.

Is it a matter of the gasses eroding the side of a .357 bullet, or is the bullet entering the forcing cone off center/canted and being damaged, where a jacketed bullet would just sort of glide into the forcing cone?
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Old 10-26-2020, 12:26 PM
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The barrel should be smaller. Slug your barrel. I use .355 jacketed and .356 lead.

Last edited by 4barrel; 10-26-2020 at 12:28 PM.
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Old 10-26-2020, 12:59 PM
needsmostuff needsmostuff is offline
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I think a couple of reasons. First is alignment as you guessed. Second is pressure is needed to expand the brass for a good seal against the chamber wall.
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Old 10-26-2020, 02:38 PM
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Jacketed bullets get a much better grip on the rifling due to a higher coefficient of friction than lead. Shooting lead bullets I've never had an issue with the cylinder throats being smaller than the bore. As soon as the bullet hits the rifling the pressure behind it will cause the bullet to obturate and fit the bore. My Performance Center .44 Magnums have .4285 cylinder throats and are very accurate.
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Old 10-26-2020, 02:42 PM
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Are you have issues with a cast bullet in your 9mm revolver? Normally any lead bullet smaller than throat diameter will give poor results; barrel leading, accuracy and often tumbling. Jacketed bullets are affected less by gas cutting/melting (blow by), and engage rifling better if smaller than groove diameter.

I would size cast bullets to the same size as the diameter as throats. I have used cast bullets running .005" larger than groove diameter with no problems, so if the gun's groove diameter is .356" that's only .004" difference (.002" more engagement). But it would help f you knew the groove diameter...
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Old 10-26-2020, 02:54 PM
mscampbell2734 mscampbell2734 is offline
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Sorry its NOT my gun, someone else posted in the revolver section he was trying to match his loads to his gun and had slugged his throats and found them to be .360.

The questions is just for my general knowledge.
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Old 10-26-2020, 05:26 PM
series guy series guy is offline
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I wouldn't think a .358 cast bullet would be very accurate out of a gun with .360 throats. Jacketed bullets would be better. I shoot cast bullets sized to .430 out of my Performance Center guns with exceptional accuracy.
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Old 10-27-2020, 12:23 PM
STORMINORMAN STORMINORMAN is offline
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Where is he measuring the throat: at the front or the rear (closest to the forcing cone/barrel) end of the cylinder? This could differ. Actually, I would think it should differ?

The 9mm uses a tapered case: some revolvers may only index on the rim...
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Old 10-27-2020, 01:30 PM
series guy series guy is offline
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The throats size shouldn't vary. The chamber can but that has no affect on throat size.
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Old 10-27-2020, 01:52 PM
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.360 would be awfully large for 9mm. I’d expect jacketed bullets to preform poorly if .360 throat and .355 barrel. .001 or .002 Difference doesn’t effect jacketed bullets much but .005 is going to mean the bullet hits the forcing cone off center by a considerable amount.

Lead is much softer than copper jackets. I think the final thought in the OP is correct. When a lead bullet hits the forcing cone off center it is much more likely to be damaged and deformed whereas a copper jacket will hold up better and self adjust within reason.
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Old 10-27-2020, 02:42 PM
STORMINORMAN STORMINORMAN is offline
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Question I'll try and put it another way...

Quote:
Originally Posted by series guy View Post
The throats size shouldn't vary. The chamber can but that has no affect on throat size.
The measurements at the front of the cylinder (after the reduction in diameter at the chamber throats) should be less than the measurement where the bullets go into the cylinder. Then there's that throat in the barrel after the forcing cone.

Yes, the measurements s/b the same for all the cylinders.

And .360 does seem to be pretty large for any 9mm application...

My question is where is the OP making his measurements?

Cheers!
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Old 10-28-2020, 11:03 AM
mtgianni mtgianni is offline
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My opinion is that a lead bullet swelling to fit a 0.360" throat then shrinking to fit a 0.356" bore is going to loose some balance. It will probably yaw at minimum and not be accurate. It would not if all was perfectly aligned but that is going to be rare. Also a 9mm bullet is short enough that it will accentuate this rather than if it were a 200 gr bullet fired out of a 38. The longer bearing of the 38 bullet will have enough behind the deformity to over come the yaw.
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Old 10-28-2020, 11:41 PM
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The proper way to measure chamber throats is to use a pin gauge. Never heard of anyone slugging the cylinder to measure throat diameter. I’m curious as to what make/model revolver this is.
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Old 10-29-2020, 12:09 PM
mtgianni mtgianni is offline
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I slugged more than a few cylinders before getting a set of pin gauges. Even now I only have the .250"-.499. Use a soft lead sinker and a light oil lube.
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